I wouldn´t be surprised if "black top" was after Timothy (Francis) Leary (see his mug photo) at http://beatlescollege.wordpress.com/201 ... -together/
The guy was running for California governor at the time, until Leary was busted. Chances are that they made use of some kind of irony since that guy was from the armed forces, and thus, perhaps he used such military crew cut.
On the other hand it may refer to the Ringo Star music style, influenced by the "bluesy" musical roots. So, the musicians that were into such style at the time used to sport some kind of flat top hairdo (stereotype aside).
That´s the line of thought here:http://www.lyricinterpretations.com/bea ... e-together
Or both things, with music and The Beatles it´s difficult to separate what were real from what was hidden in the words. In fact, at that time privy meanings in songs were the thing all over the world.