Well it is ridiculous to find a black boy/girl at the Harry Potter Prizoner of Azkaban Movie. I am sure that JK Rowling never mentioned about a black boy/girl on the hoghwart in her 7 books first 3 books (Prisoner of Azkaban is book # 3, if i am not wrong. I have read first 3 books and watched the first 3 movies). May be the movie producer try to be not racist, so he include a black girl/boy in his movie. I thought it was a blooper of Harry Potter movies
I am imagine at Hogwarts there were moslem boys/girls, christians, jews …. ha ha ha.
Note: the black men should not conduct magic, they do voodoo.