I recognize that a founder identified as a marxist, that doesn't make BLM itself a marxist organization. If a CEO is Christian, it doesn't make their company Christian, unless they choose to make a Christian company. Plenty of Christian founders have secular companies.
No where on the BLM mission statement, or website, will you find anything about installing communism. You won't find BLM claiming to be marxist as an organization. The absolute most extreme way they could be described is socialist.
And sure, they want a social safety net. Capitalism allows for that, we have social security already. It isn't extremist to ask for inclusion into the social safety net that already exists.