Black on black violence is an issue.
However, BLM exists because of the issue of law enforcement, who are supposed to be helping people, are often times unjust towards people of color and when there is a black person killed by police when they should not be protected by qualified immunity either get no repercussions or a slap on the wrist. One of the reasons for the prevalence of gang violence in the black community is that a large amount of black people do not trust the police to help them. People who are sworn to uphold the law.
Until that issue is mitigated or resolved, solving gang violence will be a harder task.
Focusing on one aspect of a problem doesn't mean you don't care about the whole problem.