Well, here's a possible reason for that... http://abcnews.go.com/blogs/entertainment/2012/03/harry-potter-star-sentenced-for-london-riots/
OKAY THIS BOTHERS ME. The character pictured is Blaise Zabini, who is an actual character in the books (though I don't remember any mention of race in the books, he could very well have been black). Originally Crabbe and Goyle were supposed to accompany Malfoy into the Room of Requirement, where Crabbe would die after (foolishly) conjuring the Fiendfyre. Because Crabbe's actor was caught growing pot (or selling it? idk), he was unable to appear in the movies anymore, so they put Blaise in his place. In the Deathly Hallows movie, Goyle is the one to conjure the Fiendfyre, and is the one to die. Blaise escapes.
I think that in Half Blood Prince it was mentioned that he was black. Which was part of the reason for the sh*tstorm some of the fangirls cause after HBP came out, since "We thought Blaise was a girl, now we can't use her/him as a self insert ;.;"
Nah, the only mention of Blaise before HBP was in Philosophers Stone, and they just said he'd been sorted to Slytherin, without mentioning his gender. Crabbe didn't have his race mentioned (so of course he's assumed white.)