Damon Root at Reason has a look at anti-black brutality from the American left. An example:

[L]et's not forget the racist history of the American labor movement, particularly the powerful American Federation of Labor (AFL). Since most AFL unions banned African Americans outright until federal anti-discrimination laws appeared in the 1960s, blacks often had to take drastic measures to break into union-dominated fields. This led many African Americans to accept dangerous work as strikebreakers—"scabs"—while the lily-white AFL walked the picket line.

In response to this unwelcome competition, AFL chief Samuel Gompers thundered: "If the colored man continues to lend himself to the work of tearing down what the white man has built up, a race hatred worse than any ever known before will result. Caucasian civilization will serve notice that its uplifting process is not to be interfered with in any such way." Along those lines, during the infamous 1892 Homestead strike against Carnegie Steel, black strikebreakers were beaten and dynamited by members of the picketing Amalgamated Association of Iron and Steel Workers.