Marxism (and economic determinism) are not inherently "non-racist." That is (in my view) a weakness of Marxism. But it is not (of itself) a reason to dismiss BLM.

As a supporter of the notion of Black lives matter (in lower case), you are doubtless an ally with a number of groups with whom you disagree on other points. That's fine. Life is complicated.

Just persevere with your own personal anti-racist agenda. If you do so honestly, you'll have no reason to apologize to or to quarrel with other groups. Let the Marxists approach the problem from a Marxist point of view; you can continue to approach the problem from a non-Marxist point of view.

There's lots of work to do. If we all keep at it, we will win.