I can recommend quite a few, since I studied at a Marxist university (Havana).

A great place to start would be Capitalism and Slavery, by Eric Williams, which studies the relationship between the slave trade, slave plantations, and the economic growth of the UK. Very serious and well done, as well as well written.

The recent book The Half Has Never Been Told, by Edward Baptist, while not formally a 'Marxist' study, is definitely a very left leaning (and extremely serious) study of slavery in the Ante Bellum US. Just as well written as the book by Williams.

Wallerstein's The Modern World System, in approximately 5 volumes, is a study of the birth of capitalism during the modern era. Not genuinely Marxist, as he has some critical theoretical discrepancies with traditional Marxist theory (such as how capitalism is best identified), but he certainly grew out of investigative traditions deeply indebted to Marxist forms of analysis. Keep in mind, he is a sociologist by training and his work is controversial for its conclusions and because (as I understand it) a few important missteps that came with him writing about periods about which he was not an expert, but overall I would say it is worth the read, if only to have an opinion about World Systems-Analysis, which has a committed academic following that has stood the test of time.

Eric Hobsbawm is probably the most widely accepted and revered Marxist historian of the last half century or so, beginning with his trilogy about the "long" 19th century: Age of Revolution, Age of Capital, Age of Empire. His book on the 20th century, Age of Extremes, is also worth a read, though he writes it as a contemporary of the major events he narrates and not as a specialist in the 20th century. His books on nationalism and rebels, respectively, have also been pretty well received. I haven't read any of his articles, I'm afraid, so I can't speak to them, but they should be pretty good given the rest of his body of work. If you're interested in how Marxist historians think, you can also read his autobiography, Interesting Times, which I found enjoyable, despite disagreeing with his apologetic arguments defending Stalinism. All his books carry as a strong undercurrent a preoccupation with 'history from below', and in general how big changes affected the little guy.

Robin Blackburn, a British Marxist, like Hobsbawm, has written a great deal about slavery in the Americas as well, such as his relatively recent book, The American Crucible.

Sven Beckert just published The Empire of Cotton, which I am currently listening to on and off in audiobook format. It seems to be a very thorough, comprehensive study of cotton's role in the formation of European capitalism, and how it affected the lives of the laborers who were exploited in Europe, Africa, Asia and America, in order to birth this new order. Definitely influenced by leftist ideas, though I am unsure (and doubt, given his methodology) if he is a Marxist in the traditional sense. Definitely an interesting read though.

I know of many more left leaning or even Marxist authors, but you specified people who have studied the oppressed and minorities of all kinds, thus weeding out (to my knowledge) people like Perry Anderson, who as far as I know have focused on the structures of power, such as in his interesting book The Lineages of the Absolutist State (Note: I am aware the book received some important criticism and as someone who doesn't specialize in Absolutist monarchies I cannot give a fair accounting to how well respected it is today, though still in use in Cuba).

Of course you are also welcome to read Charlie-boy himself: Capita: A Critique of Modern Political Economy. Some sections are, admittedly, more readable than others.