Radical blacks say that to whites: you still know nothing about how bad you were during the slavery era.
In the case of American black slavery it was White Christians who died by the thousands in the process of getting them freed. Guilt is being drilled more into white American kids, including mine whose ancestors never owned a slave, as German kids have had to endure over the war and the Holocaust. I'm sick of it too.
One thing I've used to put things in perspective when a black friend or colleague starts to kvetch in my direction is point out that some of my ancestry is Native American (Cherokee and Delaware), and that they're not the only ones with a reason to bitch about white people, AND ESPECIALLY DEMOCRATS (Trail of Tears, anyone?).
Bottom line is, if everybody quit bitching about things in the past which can't be changed (my ancestry is also Scottish and Irish ... am I supposed to march on London, or something?) and got on with being decent now we'd all be a lot better off. That doesn't mean we forget, just that we learn from the past and move on.