There aren't any black champions because they aren't as appealing as perhaps a white skinned champion, or a tanned champion, Would you like to know why?
As a designer, It's fairly hard to design noticeable changes on black, If you were to take a white champion and add many complex designs to a white champion and to a black champion, it would be very much easier to do this with the white champion, while doing the same with a black champion would hold many less worthwhile results, Not saying it can't be done, But I'm saying it's much harder and much easier. Also, Black on White looks more real, but white on black looks less real, Think of it as the shading panel somewhat, When you have a white canvas, it's easier to shade with many different colors because they show up very well, But on a black canvas, it's less easier to do the same.