In Iraq, there's small pockets of black African Iraqis named the Zanj, who are former slaves. Which made me question, since the african slave trade was so large, and far reaching, are there any other groups like the Zanj around the world? Are there any descendants of these slaves in China? Large enough to make a community?
Where did you read bout the assertion in your title question?
In the Tang Dynasty, having dark-skinned foreign servants was popular among the nobility. There was a saying in Chang'An that the best servants would be "Xin Luo girls, Kun Lun slaves". Xin Luo was a kingdom in the Korean Peninsula, and Xin Luo women were supposedly very obedient servants, so were the "Kun Lun slaves", who were Southeastern Asians and Africans. Interesting enough, these slaves were also known as the Zeng-gi, and were hypothesized to be from Zanzibar. It is very difficult to know for sure though. The Ancient Chinese seemed to identify all dark-skinned foreigners as one. This custom of keeping dark-skinned servants was no longer mentioned after the Tang Dynasty.
Sources: The Golden Peaches of Samarkand: A Study of T'ang Exotics (Edward H. Schafer)