I always that it's interesting that the majority of American-born black people that I know (myself included) have ancestors who were slaves (in my case, I have written documentation), yet I have NEVER met anyone- in person- who would admit that their family owned slaves. There is always an excuse about why it's not possible that their families would own slaves. I grew up in the South and once questioned this issue at school. I got branded a "racist," go figure. I'm now 30 and still have never come across anyone who will admit to it.
People are in denial about this issue. I think it's a combination of families covering it up, shame, denial and/or pure ignorance (ignorance meaning they have no clue it happened). But sticking one's head in the sand does not change what happened in the past. Acknowledge it and move on.