So, the original idea there is police in the United States are killing black people for stupid reasons, which is true. There are no statistics to worry about for that one and there don't need to be: just look at a handful of publicized cases and be angry.
But of course, people do like statistics. You can't be angry without statistics. (Truly, you shouldn't. It's unhealthy). Also, that framework makes a terrible export. (Which is unfortunate because Canada loves importing protests from the States instead of making its own).
So, various other progressive movements globbed on to the name, as they do, but fortunately it's a more visceral thing than, say, Occupy Wall Street, so they are at least mostly on topic. I think the real problem, which is the source of most of the recent anger (see the equally badly named and easily co-opted slogan "Defund the police"), is that police are killing a lot of people for stupid reasons.
It is still important to emphasize that black lives matter, because they do, and it's infuriating that that makes people uncomfortable. But the root cause is the United States has an unreasonable approach to policing in general, which creates as many problems as it solves. And I think if you talk to most BLM supporters, they aren't going to tell you about racial sensitivity training or hiring more black cops: they're going to tell you how the police in a developed country shouldn't act as if they're expecting a war.