"I hadn't heard any of them describe themselves as Marxist."
Yeah, rapists, child molesters, and serial killers generally don't advertise themselves as such, so why would an equally vile group (da commies) do any different?
Look here...
http://blacklivesmatter.com/guiding-principles/
...and see for yourself.
As for what I like to call 'em: intentionally paintin' with a broad brush, I call 'em 'communitarians', a grouping that includes all democrats, all progressives, most republicans, most conservatives, a great many libertarians, and a few others I'm too lazy to list...pretty much any group, or person, promoting many over one, 'we' over 'I', is -- one way or another -- in cahoots with the commies.