I have to disagree with some folks. It is possible but exceptionally rare and it isn't something that you can breed for or plan for. It is like having a horse with a grease spot, it just happens with absolutely no genetic reasoning behind it.
Unfortunately, I have no pictures but my Dad told me about a horse he rode back in the late 60's that was chestnut and had a black blaze. I've never seen anything like it but I don't doubt that he's telling the truth.
It would be about like this palomino that seems to have all his sooty coloration concentrated in his mane. It's one of those things that is extremely surprising/shocking and highly unlikely to happen again anytime soon, if ever.
I was going to say smrobs, there are things that can cause that but they are so random that the genetics of it are probably never going to be known. Sooty could, in theory, compact itself onto one location, such as in that palomino.
You could also, in theory, get a large bend-or that appears on the forehead. Chances again are really low, but it could happen.
Back on a different theory in this thread, you could try hair dye =P