I think we can all agree that the choice of models has to reflect the aesthetics of the house. Dior Homme is about 'skinny, white boys' and they suit the clothes perfectly. Of course, this doesn't mean that a non-skinny boy, a black boy or even a manly man can't wear it, which is proven by Dior's costumers and celebrities.
What's particularly difficult about commenting on the black guy in this show is the fact that it enters the race area, which is always subject to misunderstandings. I don't think Slimane should have casted a black model simply because I don't think it suits Dior's aesthetics. I would, however, always expect to see black men in Armani's shows, for example, because it suits his aesthetics.
Whether or not Dior's black boy in question is handsome or not is a matter of taste. Generally, I expect to see more 'pretty boy' types in Dior's shows, which I don't believe this guy lives up to, black or white. Anyway, it's a brave move on Slimane's part consumer-wise, and who knows, Dior might increase their sales this way.