I have a feeling that Jill Stanek, the author of the article that is the source for this thread, was not a math major.
First off, kudos to mjdonnelly, who is exactly correct - zero is the answer. They never had a chance to become a part of the population. A similar question would be “How many Catholic Answers Forum members have been killed by abortion?”
Secondly, not all of the aborted kiddies would still be alive today. Some would have died in childbirth, neglect, childhood disease, violent crime, accidents, cancer, etc.
Third, even if we accept the creaky premise that the black population should measure 49 million, then 13 million out of 49 million is only 27% killed; less than the one third claimed.