We grabbed lunch in a tiny, grungy old dive in Dundee called The New Popular Restaurant yesterday. 5 tables total, waitresses in hair nets, awesomeness! The menu had exactly this: White Pudding, Red Pudding, Black Pudding, Fishcakes, Deep-fried Sausages, or Deep-fried Pizza, all served with chips and a cup of tea. £4 each, tea included.
I asked our waitress to explain the differences in the puddings, and she sat right down at the table to do it. When she called over to the line cook about what exactly was in the Black Pudding he grinned and said that I'd have to try it before he told me. So I did. When doused with lots of vinegar, and while thinking furiously about something other than what you might be eating, it's not very bad. Not really. Also - you shouldn't really breath in vigorously through your nose. And don't be stingy with the vinegar.
He never did tell me what was in it.