This is the afterwards, it was much worse before, covered in half an inch of tar, but as I said this was with the biggest most industrial sand blaster that basically exists, so short of taking the faces completely off the bricks, not sure it will get any better. Sorry about the lighting in the photo!
My folks have had a few old farm houses in France over the years. Lime stone means that they're pretty unforgiving when it comes to old soot stains, but the softness means it can usually be blasted off.
But like you said there are some spots, like where an old chimney ran, which just don't clean.
This is the afterwards, it was much worse before, covered in half an inch of tar, but as I said this was with the biggest most industrial sand blaster that basically exists, so short of taking the faces completely off the bricks, not sure it will get any better. Sorry about the lighting in the photo!