Stanford Encyclopedia of Philosophy Race entry:
Racial skepticism holds that because racial naturalism is false, races of any type do not exist. Racial skeptics, such as Anthony Appiah (1995, 1996) and Naomi Zack (1993, 2002) contend that the term race cannot refer to anything real in the world, since the one thing in the world to which the term could uniquely refer—discrete, essentialist, biological races—have been proven not to exist.
This is the most radical theory of the ontological status of races: there are none! There are no races, not even as social constructs. But this leads to absurdity.
For if there are no races, then it is not possible that any race suppress or enslave another. But surely that is possible. Ergo, it is false that there are no races.
'Some blacks enslaved other blacks' and 'Some humans enslaved other humans' differ in their truth conditions. But there cannot be any such difference in truth conditions if there are no races. Ergo, etc.
Whether or not it is true, it makes sense to say that whites owe blacks reparations for slavery. But this would make no sense if there were no races. Ergo, etc.
The expressions 'racial integration' and 'racial segregation' have meaning and differ in meaning. But how could they if there are no races?
And how could Obama be a mulatto if there are no races, or only one race, the human race? Did he not bring whites and blacks together and heal racial divisions? How could have done that marvellous thing if there were no races?