The Bible says: “The sins of the fathers are laid on the shoulders of their children, yea unto the seventh generation.” (Actually, elsewhere in the Bible it says 3 to 4 generations. Someone obviously added them together.) Now, a generation is defined to be between 20 and 30 years ... so, using the average of 25 years, this means the sins can be transferred to issue for about 175 years.
Now, the Emancipation Proclamation was issued in 1862, so the sin of slavery is due to expire in around 2037 according to the Good Book. After that, any possibility of Devine punishment to the descendants of sinful slave holders should finally disappear.