I never imagined the Qartheen as being black...aren't they even described as being really pale in the books?
And why would they rename Asha? Weird...
Qarth is a massive city, with lots of trade, and a large population of slaves. Ethnic mixing would be common. It's not a stretch by any means for Xaro Xhaon Daxos to be black; I can't remember quite how he's described in the books. All I remember is he has lots of jewels on his nose or something.
And I'm assuming they renamed Asha to make it dissimilar from Osha. Not sure it was necessary, but it's not like her name has to be Asha for plot reasons. Yara works just as well.
They've also cast someone as "Alton Lannister", who I'm guessing is going to be a mix of Cleos Frey and Kevan's sons.