There was also another black slayer in season 7 when they had had several slayers at once.
I was a big fan of Buffy...collected all the season on dvd as well as the spin off Angel.
I think Joss Whedon is trying to relive his glory days when he was in his prime with a number one show and a busy casting couch. I don’t think things have been going to well for him since his wife called him out for being a pervert.
They ought to go back to the 1970s slayer. She’s black, lives in New York City in its darkest days, single mother, and kills vampires.
http://buffy.wikia.com/wiki/Nikki_Wood