For every death of a black by a police officer, there are over 20 from black on black violence.Why is this ignored?

It's not ignored. Lots of people are concerned about violence in US cities. 

They're trained to do a job, sometimes they aren't trained very well, and sometimes the cops are just bastards. Sometimes the laws and sentences are unfair and racially motivated, and it doesn't matter who kills people, it matters why they're dying, and usually most crime is about money for the drugs the CIA sells. 

HARLEM -- New York public leaders, community organizations and residents gathered Sunday to celebrate the 42nd annual African American Day Parade in Harlem. One focal point of the march was to attenuate the looming violence in neighboring and citywide communities

Because police officers should be held to higher standard than criminals.

It only gets attention if the cop is white

Same can be said for whites. I was just looking at the stats last night, and I know this for a fact.

Submit a call to action. If it were a big deal, it would be protested or should be. Lead the charge!