This character, at least to me, seems to be inspired by Hadrian of Canterbury, "by nation an African" (Bede, Eccles. 4.1) that came to England to serve as an abbot. He was also accompanied by the monk Theodore, "born at Tarsus in Cilicia" (ibid.), who became Archbishop of Canterbury. While this particularly case comes from the late-seventh century, it clearly demonstrates how interconnected Europe and the Mediterranean were, and how travel was relatively common, at least for clergy, with Hadrian being said to have made the journey from Naples through France twice before coming to Britain (ibid.). That Alfred was sent to Rome when we was relatively young suggests that travel about Europe and to the Mediterranean remained particularly common two centuries later duirng the 'Viking Age' (Anglo-Saxon Chronicle, 853). There is also potential evidence for Anglo-Saxon goods being found in West Africa, possibly traded along old Roman trade routes or new routes opened up with the expansion of the Arab world (see here). While it is impossible to say whether these goods were carried by Anglo-Saxon traders or intermediaries, it does tell us that there were connections between Britain and the Eastern Mediterranean and Africa that goods and people could travel along. [EDIT: I'd also like to mention the Byzantine silver bowls and silver platter found in the Sutton Hoo burial as evidence of long-range trade between Anglo-Saxon England and the Eastern Mediterranean, suggesting a more connected world than imagined in the popular conscious.]

As we have seen, the coming of the Vikings did not interrupt connections between Britain and the Mediterranean, what with Alfred's pilgrimage to Rome. Indeed, the coming of the Vikings, themselves being maritime explorers, might have intensified such long-range contacts. The Fragmentary Annals of Ireland attest to a Viking raid on Mauritania (North Africa), during which the Viking raiders captured a great many of them, bringing a "great host" of them to Ireland as slaves. This event is attested in Al-Bakrī's Book of Roads and Kingdoms from the eleventh century, but based upon earlier works (here), and in The Chronicle of Alfonso III (entry 27). While these slaves are said to have been taken to Ireland, it is not impossible to imagine them being traded also in England, or Vikings from England having gone on the raid and returning to England, and not Ireland, with their captives. From there, it is certainly possible that these slaves came to be involved in the communities they were brought into, possibly serving in churches, and becoming priests in turn. There is even evidence of burials for people of African descent in contemporary Britain at this time (here), with one from York suggesting, based on osteological analysis, to have had a hard life with dietary deficiencies, suggesting the man was poor or a slave (here).

Thus, while the only attested clergyman of African descent comes from the seventh century, preceding the historical setting of The Last Kingdom by two centuries, there is certainly evidence that Africans were present in ninth century Britain. It would be easy to dismiss those in Britain as all being slaves, but we do not know how all the people in Britain of African descent came to be there. They may have come as slaves, as the Fragmentary Annals of Ireland suggests, but they might also have come via the connections of the church, just as Hadrian of Canterbury and Theodore of Cilica did, or they may have come as traders to list but a few possibilities. A black priest in ninth century Britain is conjecture, but most history is, and it has greater precedent than most historical conjectures. As with most of the minutiae of every day life throughout history, we cannot state for certain if anything ever did or did not occur. There likely were black people in ninth century Britain, and some of them might have been priests, but how many is impossible to say.