Haven't used the black version, have used fine white and the general purpose stuff. the former was always the most useful, but I always found milliput rather stiff and unyielding to work with, and pretty hopeless for filling anything more than very small areas.
For a good epoxy filler I recommend Easy Fair by Reactive resins, which spreads and sands like polyester filler, but without the shrinkage, plus it's fully waterproof once set. Takes much longer to cure than polyester filler though.
If you want to mould parts, I think you'd be better off with polyurethane resin, which is the consitency of water when poured and cures pretty quickly.