"BLM" in this instance is Bureau of Land Management... the people upon whose land the Bundy's decided to graze their cattle for free.
The BLM is tasked with protecting the commonly-held lands in America (national parks, open spaces, unclaimed spaces, etc). So they ask for cattle ranchers to pay a fee to graze their cattle on public lands.
The Bundys didn't pay, so the BLM went in to collect.
Could have gone all Waco, and didn't, because everyone was standing around thinking, "Shit. This could go all Waco, and shit." And nobody pulled the first trigger.
Black Lives Matter has no role in this, other than choosing a three-letter descriptor that had already been in use for decades. (See also the WWF and the WWF.)