It is petty, true, but my mother actually pointed this out to me. Yes, Obama is technically our first black president, even if he is also white, but it's also funny how we easily call him a black man, but can be hesitant to call him white. It's not even about skin tone, fully (lots of people with two black parents have light skin), but it is tongue-in-cheek humour.
Trust me, this is coming from a fairer-skinned (sorta) person who has been joked about as being "not black."