They'll use the question, why were the slave majorily black -- a simple answer. The trade, though outlawed on paper, still thrived and so ships as late as 1859 were taking slaves from Africa and importing them. And attempts to indict the slave traders went south.
If the trade would have truly ended, the descendents of our slaves today wouldn't be purely black but more like Al Gore. Logic dictates that slaveowners raped and bled slaves. So the fact that there are pure black descendants of slavery proves that the trade went on even after wards.