Abram Petrovich Gannibal (sometimes called Hannibal), the great-grandfather of Russia's national poet Alexander Pushkin was a black slave who was captured as a child and rose to become a nobleman in Russia.
He was brought to the court of Peter the Great and was very liked for his charme, wit and intelligence. They sent him to study in France and Gannibal became a highly educated military engineer, general and governor of Tallinn.
Edit: I'm sorry just realised you said before the 18th century :/
I don't know of any from the imperial roman era but the during the renaissance, the Medici family was the closest thing Europe had to an Emperor since the romans themselves.
"Alessandro de' Medici (July 22, 1510 – January 6, 1537) called "il Moro" ("the Moor"), Duke of Penne and also Duke of Florence was ruler of Florence from 1530 until 1537. Though illegitimate, he was the last member of the "senior" branch of the Medici to rule Florence and the first to be a hereditary duke."
"Born in Florence, he was recognized as the only son of Lorenzo II de' Medici (grandson of Lorenzo de' Medici, the Magnificent), but many scholars today believe him to be in fact the illegitimate son of Giulio de' Medici (later Pope Clement VII), nephew of Lorenzo de' Medici, the Magnificent. Historians (such as Christopher Hibbert) believe he had been born to a servant of African descent who was working in the Medici household."
http://en.wikipedia.org/wiki/Alessandro_de%27_Medici,_Duke_of_Florence
Many of the moorish rulers of Spain (8th c. to 12th c.) might be considered "black" ; the general of the Umayyad conquest, Tariq ibn Ziyad, was of Berber origin, for instance.
Medieval art sometimes depicts Moors as black, sometimes not, often depending on political and/or religious context.
Although 18th c. (and so a bit late), Ignatius Fortuna was a prominent "Kammermohr" (court-moor, a sort of page position) , and became an important figure (and possible advisor? not sure) in the court of Countess Palatine Francisca Christina of Sulzbach.
