Of course that's true. The problem is that some people don't place much value on certain lives, so while society has made it crystal clear it places a high value on, say, white lives, it doesn't place near the same value on black lives. BLM is an attempt to directly address that, to bring attention to the fact that black lives aren't equally valued by society even though they should be.
Discussion
Replacing one lie with another isn't a great idea.
The truth isn't that black lives matter, it's that racist lives generally don't.
People who went "black lives matter" instead of "racist lives don't matter" were focused on money instead of truth, and being obsessed with money seems to psychologically fit with being racist in the humans of the world I've seen.
Yes, but then who decides what and who is a racist?
The average BLM supporter thinks racism = ethnic prejudice so they would mean ethnically prejudiced people. Instead of pretending to think there's confusion over whether anyone values black lives, they would be telling the truth about their desire to prove it's impossible to rule out warfare and killings against ethnically prejudiced people.
The average BLM supporter is also ethnically prejudiced, and also pretends humans aren't one race, which is the real racism. They would rather say "black lives matter" because if they admitted "racist lives don't usually matter" they'd be admitting they're in a pointless fight killing people just like them and getting killed for the same reasons they kill other people for.