There's the notion of BLM, of which only sociopaths would disagree, then there's BLM the organization which is exactly how you described it.
The naming of this organization was actually a stroke of genius. Only the crazy edge cases would refute it's name but served as a Trojan horse for an ideology that is inherently un-American. If you disagree with them the immediate reaction has been "you don't think black lives matter?" This is a brilliant move.
The left has gotten clever with their naming, similar to the conservative movement in the 90s under the advisement of George Lakoff ("death tax", etc).
It's a Motte and Bailey, by design (which I seem to end up pointing out a lot on HN, apology for the repetition but it's a common and important pattern).
The core BLM organization are subversive Marxists. Their tenants are extremely un-American and probably not shared by most of the people marching under their banner. When you point this out, they can retreat from the Marxist Bailey to the racial equality Motte and attempt to shut down criticism.
This pattern of equivocating between subversive ideas and basic liberal tenants is extremely damaging for the liberals accidentally aligning themselves with extremists, but this smokescreen tactic has existed longer than most any HN posters have been alive. This is the proverbial "Useful Innocents" (and its more pejorative alternative).
The naming of this organization was actually a stroke of genius. Only the crazy edge cases would refute it's name but served as a Trojan horse for an ideology that is inherently un-American. If you disagree with them the immediate reaction has been "you don't think black lives matter?" This is a brilliant move.