People fighting for black lives aren't hiding bad things that happen to white people. The reason Duncan Lemp doesn't have posters up about him isn't because black activists against police violence haven't fought for him, it's because both black and white activists against police violence haven't fought for him. And as white people still make up the majority of people shot by police[1], far more white people than black failed Duncan Lemp.
I find it to be a fantasy that somehow, if people fighting for black lives would emphasize the suffering of white people, it would suddenly turn the white people who are hostile or ambivalent about this stuff into supporters. Again: you can't get white activist gun owners to defend a licensed gun owner who was surprised and shot in his own home by police executing a no-knock warrant that wasn't for him. The NRA supported gun control to criminalize the Black Panthers. The Black Panthers were an absolutely straightforward open carry movement, with one important difference.
edit: I meant to exclude Reason-style libertarians from this. They're scrupulously consistent.