I have wondered for a while if this schmutz (to stay in keeping with the technical terms : ) is low grade burning from urine splatter. It seems to be more common among geldings tho not exclusive to. I know of mares with similar issues but it seems to be inside the hind cannons as opposed to in front, as with a gelding…. mostly just thinking out loud : )
Hunterjumper (I love your technicality, I get it :D) offers good advice on keeping things clean and treated.
Sure there's right and wrong but mostly there's just a whole lotta different.