You could speak to Strippers of Sudbury who are less interesting than their name, but do have a variety of specialist products that remove different paints etc.

I have not used them but I know someone who did get something from them to strip the old thick black stuff off his beams.

I painted some beams brown once... they looked rubbish, like big chocolate flakes...

Best to strip back to their natural state or, failing that, paint them the same colour as the adjacent wall/ceiling so you don't notice them.

Oak beams can be very dark on their own (especially old ones). If they also come from stables or cattle transport wagons (re-used) you might have 'smoked' beams, where the ammonium vapour from cattle 'pee' has reacted with the tannin acid in the Oak. This dark colour goes very deep into the wood, and trying to get back to bare wood might render you beams too thin.

Try the discussion forum at Period Property website, and ask how to deal with them there--you'll get good advice. Painting and aggressive stripping are not the best way to go with lovley old timbers: be gentle with them, and they'll end up looking gorgeous.

Been there, done this. Naturally aged oak looks Ok, but if already painted black, then probably will not strip off as will be well soaked into the timber. The colour you need is a very dark brown matt vinyl emulsion. It is I think a standard BS colour, 08B29, and is sold in the Leyland paint range as PEAT. Looks perfect. The pigment in the paint is very heavy, needs massive stirring up, and the slightest splash is difficult to remove - and the brushes almost impossible to wash out.