In short, white wax is for vehicle paint that primarily need defect removal, with a lesser emphasis on polishing oils, while still having a high quality long lasting wax.
Where as, black wax is for vehicle paint that primarily needs polishing oils with a lesser emphasis on defect removal, while still having a high quality long lasing wax.
I see these as higher end AIOs with an emphasis either on defect removal, or paint conditioning with polishing oil.
We also need to remember that most of Meguiar's products and marketing is designated towards the average Joe, not a highly knowledgeable enthusiast. It's simplified to make the choice of "What product do I need" easier for the average person.