The most obvious example is the class of slaveholders. Slavery is an unjust system. But it is also an inefficient one. It was no coincidence that the North blockaded and invaded the South, and not vice versa. A society based on slave labor will never be as industrious as one based on free labor.
Modern blacks are certainly poorer because of slavery.
But so are modern whites. Rather than relish in historical episodes of theft facilitated by governments, let us get on to the task of eliminating present ones.