The term "subspecie" implies that the two populations cannot interbreed unless artificially forced to. E.g. two populations living in isolation. Sometimes it also implies that the offspring has lower fitness (e.g. half of the descendency is sterile). Usually two subspecies show phenotipical differences, but it's not enough.

The term doesn't apply to humans because through all history black people has been in contact with non-black people, and interbreeding without any trouble. Having a bit more melatonine in your skin doesn't make you a new subspecie.

But if your head size is huge (like Neanderthal), so huge that half your iterbreed newborns die or kill the mother, you are on the path of speciation but still the same specie because interbreeding is possible.