I agree with you that the colonialism and capitalism imperialism killed millions of persons. I also believe that counting casualties is very difficult. I think that if you apply the same criteria to calculate the deaths due to capitalism and stalinist communism in 20th century you will probably reach to something like 80-100 millions for each system (depending on how you count NAZI casualties). But I've quote the Black Book of Communism in a different context. I was trying to say that a well done research on the casualties in USSR will reach something like 15-20 million (even De Long agrees about this).
These number are still very high but there is no comparison with NAZI atrocities. We must also consider that many of deaths caused by capitalism happened during centuries XVIII and XIX (slaves traffic, Ireland famine, deaths during the industrialization). This process of primitive accumulation was exactly what happened in USSR from 1929-40. If you read the history of English industrialization in the first half of XIX century, you will find many similarities with Stalinism. But the English were able to transfer suffering to their colonies while Stalin had only the Soviet people to exploit. See Eric Hobsbawn's "Age of Revolutions" and we learn that the worst places for a peasant live in 1800-48 were Ireland, India, England and Belgium. The same probably was true for the USSR in the 30's. A Black book of the capitalism would also be a whorty adition to those who studies Democides (to quote Rummel)
Alexandre