I think you are confused about what's written in our breeding guide. What it says is if a MALE Aggron has Dig, it can be bred to another pokemon. Only the male pokemon passes down moves.
You breed the Aggron with a female pokemon (e.g. Charizard), and you get a baby of whatever the female is (e.g. Charmander), but it knows Dig that the male had.
But if you taught the TM to a female Aggron, now you can't pass it down through breeding because it is female. You need to "sketch" it onto a male Smeargle, then breed the Smeargle with a female pokemon, and the Smeargle will pass the move down because it is male.
(Note: the TM example isn't necessary in Black/White because you can use TMs multiple times. But the breeding works the same.)