African slaves were certainly present during the Tang Dynasty. At this time, African slaves were used in Arabic and Indonesian kingdoms, and there are records of Arabic delegates arriving in the Tang dynasty court with African slaves. Tang dynasty literature and plays indicate that some African slaves were eventually given employment in Chinese households.

One famous example is the story of The Kunlun Slave, in which a capable African slave rescues a beautiful girl for his master.

The literary references likely reflect awe at the sight of a few slaves rather than a widespread importation of slaves - the slaves in the stories often have mystical powers, and black skin is sometimes referred to as having been achieved through the application of a black laquer.

Around three hundred years later during the Song dynasty, Chinese records reflect that African slaves were a common sight in Canton, and that most rich Cantonese households kept African slaves. This practice was not widespread elsewhere in China.

The Africans do not appear to have been treated well in Canton - they are referred to in the records as animals, capable of understanding but not speaking "human speech". They're noted to be accomplished divers, and are employed caulking ships and as doormen.

Check out Philip Snow's book The Star Raft, which has a brief discussion of the above, but is also an excellent book on the Chinese experience of Africa in general.