The insult to black people to tell their stories using white faces doesn't bear thinking about.
In an ideal World, it wouldn't matter. There's nothing intrinsically black about Aliya's story. She started to carry a knife at the age of twelve. She saw one of her friends stabbed to death with a knife. She vowed never again to carry a knife or associate with people who do. That story could apply equally to a black or a white girl (or boy) and the message has nothing to do with race.
I guess it's not an ideal World though and people do seem to like to find examples of racism everywhere, so they have to be careful.
given that she's black, one could take it to imply that it's usually black people who carry knives with aggressive intent. What do people think?
I think you needed to look at the whole campaign which includes posters of actors of varied races and genders.