If by Black, you mean, Whitest White guy who's 100% white in color and heritage. Then yes, Leo is in fact Black.
I always felt his sprite looked white but then I read he was black and after seeing his portrait I just stuck with it as he does look dark skinned but it's rather hard to confirm it. He's certainly not the "Whitest White guy" lol- that would be the horrible characters in Advent Children. They make albino Setzer looked tanned. Did Square spend all their money on the SFX that they couldn't afford colours and had to make do with shades of white and grey?

Somebody mentioned that Sazh looks about like Lando from Star Wars. That's fine with me! He was awesome and didn't fall into the jive talking stereotype that Barret did. Do any of the announced characters so far use a sword or anything though? It all seems to be guns.