And what scientific theories are these categories part of?

I really don't think science has much to do with the bulk (or strength) of objections you will get on this subject. You're doing yourself no good by continuing to argue about it. Even the terrible arguments made against you will receive positive support by virtue of being sandwitched between two of yours - reading need not be involved.

It is probably better to make the ethnic-group references a bit more specific than a two category split. It is fairly clear what 'black/white' labels refer to in countries with two clear dominant ethnic groups of appropriate melanin levels but less useful if trying for a worldwide reference. Then the references become more ambiguous. I suggest making your stand somewhere a bit more secure than the 'black' word. Your main point is a bit deeper than that.