I was black affronted by his clarty beets. I was embarrassed by his muddy boots.
I’ve nivir bin so black affronted as fan he bared his airse. I have never been so embarrassed as when he bared his backside.
Black affronted is probably derived from the act of covering your heraldic shield (affronty is a heraldic term) in order not to be recognised. Scots knights did this as they fled from Methven, in order not to be subsequently accused of being supporters of Bruce. Source: Robert-Low.com
another probable derivation comes from New lanark, a cotton mill operated on humanitarian lines by Robert Owen. Above a worker’s machine there was a wooden cube. Overseers would turn the cube to show the quality of work. The black front of the cube was shown when work was substandard. Hence this use of embarrassment as a social control is a more likely meaning of black affronted. Is there any use of the term known before 1800?
Another possible version could simply be the literal meaning of the two words, black as in negative and affronted as in offended/embarrassed. As far as we know it generally relates to embarassment.
As in a nasty trick on a person could vaguely be described as black magic, once again black standing for negativity, and magic as trickery. It is slightly different , but if somebody was very formal/literal in mind or speech, as they often seem to be from old text, it makes sense.
So rather than just being slightly embarrassed, they were properly/doubly so. Black in this context simply means very