Well, in the US they go by the "one drop" rule. So if you are one quarter African blood, then you're black. However, most MLB African American players are easily > 50% African blood.
The tricky part is when you factor in the "Hispanic" (or whatever word you want to use) MLB players from outside the US. Puerto Rican, Dominican, Colombian, Panamanian, Cuba... a lot of them have a great portion of African blood.... ranging from almost 0% to almost 100%.
The odd thing is that for some of them their number one ingredient will be "white guy" to put it crudely.
I would imagine this is the case for the Mexican players in the MLB. They are majority white genetically. It is easily their greatest DNA component. And it has to be because you have to have a certain size to play in MLB.
Genes don't lie but cultural brainwashing does.
A little off topic.