The first thing that should be understood is that depictions of Satan or the Devil, and understandings of who and what that name and figure personified and meant have varied considerably over time. So folks in medieval Europe would not have necessarily seen the devil as basically human with red skin, a bifurcated tail, little horns on his head, cloven hoofs instead of feet, a black goatee and mustache, and carrying a pitchfork - but neither would any of those individual attributes necessarily been out of place. One of the very general things that can be said about depictions of the devil in medieval art and literature is that they were generally eclectic, and not always immediately monstrous, at least in folkloric appearances.

The idea of the "Black Man" as Satan, or vice versa, was something that gained popularity in the confessions of the European Witch Trials. Whether this was a reflection of genuine medieval folk beliefs or a construction of the inquisitors that massaged the stories to better conform to the form that they wanted to hear isn't clear. You can see something of this in Margaret Murray's The Witch-Cult in Western Europe (which should not be taken by itself as history, but it is sometimes convenient a gauge of popular understanding in the early 20th century):

John Walsh of Dorsetshire, 1566, described the Devil, whom he called his Familiar, as 'sometymes like a man in all proportions, sauing that he had clouen feete'. The Lancashire witch, Anne Chattox, 1613, said, 'A thing like a Christian man did sundry times come to this Examinate, and requested this Examinate to giue him her Soule: And in the end, this Examinate was contented to giue him her sayd Soule, shee being then in her owne house, in the Forrest of Pendle; wherevpon the Deuill then in the shape of a Man, sayd to this Examinate: Thou shalt want nothing.' Elizabeth Southerns of the same Coven said that 'there met her this Examinate a Spirit or Deuill, in the shape of a Boy, the one halfe of his Coate blacke, and the other browne'. To Margaret Johnson, one of the later Lancashire witches, 1633, there appeared 'a spirit or divell in the similitude and proportion of a man, apparelled in a suite of black, tyed about wth silke pointes'. The Yarmouth witch, 1644, 'when she was in Bed, heard one knock at her Door, and rising to her Window, she saw, it being Moonlight, a tall black Man there'. The Essex witches, 1645, agreed very fairly in their description of the man who came amongst them: according to Elizabeth Clarke he appeared 'in the shape of a proper gentleman, with a laced band, having the whole proportion of a man.... He had oftentimes knocked at her dore in the night time; and shee did arise open the dore and let him in'; Rebecca Weste gave evidence that 'the Devil appeared in the likeness of a proper young man'; and Rebecca Jones said that the Devil as 'a very handsome young man came to the door, who asked how she did'; on another occasion she met the Devil, 'as shee was going to St. Osyth to sell butter', in the form of a 'man in a ragged sute'. There are two accounts of the evidence given by the Huntingdonshire witch, Joan Wallis of Keiston, 1646: Stearne says that she 'confessed the Devill came to her in the likenesse of a man in blackish cloathing, but had cloven feet'. Davenport's record is slightly different: 'Blackman came first to her, about a twelve-moneth since, like a man something ancient, in blackish cloathes, but he had ugly feet uncovered.' The evidence of the Suffolk witches, 1645-6, is to the same effect; Thomazine Ratcliffe of Shellie confessed that 'there came one in the likeness of a man.—One Richmond, a woman which lived at Brampford, confessed the Devill appeared to her in the likenesse of a man, called Daniel the Prophet.—One Bush of Barton, widdow, confessed that the Devill appeared to her in the shape of a young black man'. All the Covens of Somerset, 1664, were evidently under one Chief; he came to Elizabeth Style as 'a handsome man'; to Elizabeth Style, Anne Bishop, Alice Duke, and Mary Penny as 'a Man in black Clothes, with a little Band'; to Christian Green 'in the shape of a Man in blackish Clothes'; and to Mary and Catherine Green as 'a little Man in black Clothes with a little Band'. To the Yorkshire witch, Alice Huson, 1664, he appeared 'like a Black Man on a Horse upon the Moor', and again 'like a Black Man upon a Black Horse, with Cloven Feet'. Abre Grinset of Dunwich, in Suffolk, 1665, said 'he did appear in the form of a Pretty handsom Young Man'. In Northumberland, 1673, Ann Armstrong said that 'she see the said Ann Forster [with twelve others and] a long black man rideing on a bay galloway, as she thought, which they call'd there protector'. The Devonshire witch Susanna Edwards, 1682, enters into some detail: 'She did meet with a gentleman in a field called the Parsonage Close in the town of Biddiford. And saith that his apparel was all of black. Upon which she did hope to have a piece of money of him. Whereupon the gentleman drawing near unto this examinant, she did make a curchy or courtesy unto him, as she did use to do to gentlemen. Being demanded what and who the gentleman she spake of was, the said examinant answered and said, That it was the Devil.' In Northamptonshire, 1705, he came to Mary Phillips and Elinor Shaw as 'a tall black Man'.

Murray was cherry-picking her sources, so don't take this as an absolute census of accused witches in agreement as to what the devil looked like! But it does show that for folks that wanted to look through the trial records, there could be construed the idea that the devil came dressed in black, or was a "black man" - whether they mean that in the sense of an individual that was Ethiopian or Moorish in appearance or literally "black" as from coal isn't clear...and that goes for more than Murray's selection. Montague Summers in his History of Witchcraft and Demonology (who also should be taken with a grain of salt) talks about how witches are sometimes conveyed to Sabbats "mounted on a sticks or a broom, sometimes on a sheep or a goat, sometimes by a tall black man." and that at the Sabbat "The Sorcerers worship the Devil who appears under the form of a tall black man, or as a goat. They offer him candles & kiss his posterior," and here Summers is, like Murray, citing and synthesizing from the witch-trial transcripts in his interpretation. For a more scholarly (but less accessible) discourse, check out "Men in Black: Appearances of the Devil in Early Modern Scottish Witchcraft Discourse" by Joyce Miller.

Such depictions were influential in popular culture. H. P. Lovecraft in his story "The Dreams in the Witch House" was drawing on Murray when he wrote:

The evilly grinning beldame still clutched him, and beyond the table stood a figure he had never seen before—a tall, lean man of dead black colouration but without the slightest sign of negroid features; wholly devoid of either hair or beard, and wearing as his only garment a shapeless robe of some heavy black fabric. His feet were indistinguishable because of the table and bench, but he must have been shod, since there was a clicking whenever he changed position. The man did not speak, and bore no trace of expression on his small, regular features. He merely pointed to a book of prodigious size which lay open on the table, while the beldame thrust a huge grey quill into Gilman’s right hand. Over everything was a pall of intensely maddening fear, and the climax was reached when the furry thing ran up the dreamer’s clothing to his shoulders and then down his left arm, finally biting him sharply in the wrist just below his cuff. As the blood spurted from this wound Gilman lapsed into a faint.

I can’t speak to the depiction of Satan as an Ethiopian, but I wouldn’t be surprised in the least. I can say, as an early modernist and early Americanist, that Satan was often depicted as an “other,” a member of a group feared or poorly understood, who was often culturally as well as physically different from the Christian European group being discussed.

This extends to his acolytes. Witch accusations were typically leveled at those on the margins in local communities. Even exceptions, like Puritan minister George Burroughs, stood out in some way that marked them as exceptional in a way that drew suspicion. In Burroughs’ case, he was abnormally strong and managed to survive a Wabanaki raid that destroyed most of the settlements around York, Maine.

More broadly, groups like the Jews and Native Americans were thought by a number of writers in the early modern age to be aligned with Satan. A popular idea in English Protestant theology is a duality in which anything God does has a Satanic antipode. Thus, if God has a chosen people, so does Satan. If God leads his chosen people to a promised land, Satan must do the same. So while any number of European colonizers thought converting the Native Americans was an easy and worthy goal, a large number also saw those people and their land as intrinsically disordered and evil (after all, since they weren’t mentioned in the Bible, perhaps they weren’t in God’s light).

Regarding Satan specifically, the Puritans during the Salem witch craze described him as a “tall black man in a tall hat.” By black, they were not referring to African. The descriptions we have clearly point to Satan as a Native American in their mind. Satan often reflected the fears and prejudices that Europeans possessed, and this case was no different. While the overwhelming majority of the Salem accused were English, the supposed leader of the accused was their physical enemy, at home in a land they did not control.