Well, there is such thing as national unity and, therefore, obligations. We have this discussion in Gemora about Egyptians suing Jews in greek courts for stuff we took out and Jews countersuing for slavery. So, the subject itself is legit. Did white Americans compensated former slaves sufficiently? If not accounting for deaths and sufferings, then probably the appropriate measure would be – were they given sufficient property comparable to current West Africans, such as Liberia (that was founded by freed American slaves).
