I don't know which came first, Wallace or Black Wally.

Technically, Wallace appeared first in the comics. But the show had already casted the West family so that influenced the New 52's take on the character.

So, I think DC wanted to ve culturally diverse and at the same time didn't want to affect Batman or Superman.

It's an interesting situation.

To bring Barry back to prominence in the comics, DC had to kick out Linda Park and the twins along with Wally. The Flash mythos became less diverse as a result of going back to Barry, especially during the New 52 where Linda and the kids didn't even exist.

So the lack of diversity was only a problem at the time because DC/WB made it one. The changes to Iris and the West family in outside adaptations (along with Wallace) are because DC/WB got rid of the actual diverse characters in the grand scheme of things.