In a 1962 letter to Engels referring to a socialist competitor, Marx wrote "...he, as is proved by cranial formation and his hair, descends from the Negroes of Egypt, assuming that his mother or grandmother had not interbred with a nigger. Now this union of Judaism and Germanism with a basic Negro substance must produce a peculiar product. The obtrusivenessof the of the fellow is also nigger-like." There are more quotes from both Marx and Engels in this vein but hopefully you get the drift. The racial references are in the source and if anyone is offended, I can only point out that they are the words used by Marx and Engels.
Given the apparent glee with which Zinn, et al, point at the slave owning clay feet of some of the Founders, I wonder if they will also note that the founders and authors of The Manifesto of the Communist Party" were not entirely pristine humanists with unaldulerated love for all mankind.

In Nathaniel Weyl's 1979 book "Karl Marx, Racist", Marx, referring to the annexation of California, is quoted as saying "Is it a misfortune that magnificent California was sesized from the lazy Mexicans who did not know what to do with it?" Marx's partner Engels added, "It is to the interest of its own development that Mexico will be placed under the tutelage of the United States." Apparently, neither of these founder of modern Communist ideology had much regard for Mexicans.