Other then the recent arrivals from Africa (not involved in US slave-trading anyway), practically all african-americans in US have white blood in them. Average for the country is about 25% with south-east the lowest at 4-7% and gradually increasing north and west to above average.
That statement is based on the frequency distribution of a few dozen gene alleles that are exclusively present in people of sub-saharan african descent living in africa and not in caucasians and vice versa. Those exclusive marker genes are the result of a natural genetic drift that have occured since the sub-saharan africans and caucasians split from their common anscestor.
Any "black" person with any white ancestry in his past would not have been born if the slave trade did not bring his ancestors together to conceive a mixed-blood ancestor. No mixed-blood anscestor - no one would be here to demand reparations!
Besides the fact that all those mixed-anscestry blacks are descendants of slave-owners too!
The same applies to current "victims" of nazis or any other historical event. That is why it makes sense to consider liability to immediate victims and their clear legal heirs rather then general "historic" heirs.
The time is not a factor here - a treasure stolen from Egypt by Brits should be returned to Egypt because it would have stayed there and Egypt would be the same.
The romans, on the other hand should not be held liable for the destruction of Judea to the modern jews since if they did not do so, we would not have modern jews as we know them.
StSanta - vikings did raid at first. Then they settled eveywhere - in Russia, France, Ireland and the rest of Europe and assimilated into the native populations. Rather then one-time harassers of our ancestors, they
are our ancestors. But if you still feel you owe something, send me a check and consider your burden lifted.

miko