How does Malorie Blackman have racism?

She really doesn't. But in her books, she portrays racism against White people as satire to the racism against people of color to make a point. Some may misunderstand and think that makes her racist, when she is merely depicting racism. The idea is to try to get a White audience to understand what racism against other groups feels like.

This strategy is much like an episode of Oprah. People with green eyes were singled out for mistreatment before the show and in the early part of the show. Then the speaker got up as an "expert" and said that eye color correlated with lower IQ, ruder behavior, etc. Then the audience members of the "privileged" group got up and said that had to be true, that the green-eyed people couldn't be quiet, etc. Then the speaker let everyone know there was really no difference and that this was just an experiment. The green-eyed people acted according to what was expected of them. There was no inherent differences, but unequal treatment reinforced negative behavior, and negative behavior reinforced the notion that the mistreatment was justified. So thus this is how racism works.