For anybody not super familiar with US government departments and confused by reading this after seeing other headlines coming out of the country in recent years, BLM in this context is "Bureau of Land Management" and not "Black Lives Matter".
The BLM is the federal department that oversees most governmental land that isn't managed by some other department. Broadly speaking, this is land which isn't run by a local/state/tribal government, may not have any particular use designated, and is (usually) uninhabited - or at least intended to be.
While a lot of BLM land is uninhabited, it is not all like that and not necessarily intended that way either. For example, the entirety of California's coastline is BLM land.
The BLM simply functions as the kitchen sink of land management at the federal level, catching everything that is not explicitly assigned to other agencies.
This is a very misleading statement. The area off the coast of California constitutes the California Coastal National Monument. The actual land mass involved there is a mere 8800 acres and nobody lives there. The other national monuments in California add up to 2.5 million acres. BLM's total holdings in California are about 15 million acres, of the state's 105 million acres. Most of what the BLM holds aside from national monuments are grazing lands that they lease out.
Indeed, for reference, truly vast swathes of the rural western states are federal government land. Not all is BLM, some is national park and such, but same concept applies for zero population in a census block.
The BLM is the federal department that oversees most governmental land that isn't managed by some other department. Broadly speaking, this is land which isn't run by a local/state/tribal government, may not have any particular use designated, and is (usually) uninhabited - or at least intended to be.