I've just tried to strip my beams using paint stripper and heat gun. bith methods took off the black beam paint no problem, however, underneath was some horrid dark brown stuff which will not budge for anything. Given there are so many other pressing jobs around the house, I'm going to repaint the beams in a light colour until I have the time to strip them properly - an extra layer of coloured paint will not make any difference.
But seriously, do be careful in trying to find a solvent for black bitumen-based paint. You wouldn't be the first person to find that the solvent only makes things worse by dissolving the bitumen and allow it to soak into the surface of the wood.
Strippers of Sudbury have a product that they say will remove bitumastic paint. I haven't tried it, although I have used another of their products, which was very good.
Have a look at the "I want to remove..." section of their website.