I'm most familiar with the white Blazes on the tree along the Appalachin Trail. Blue blazes at trail junctions off it lead to look outs or water sources.
These are for sure NOT a trail. They are Forest Service property line blazes. Some have markings inside the blaze, that are "SOMEWHAT" like some roman numerals. They might mark off a given distance, or change in compass direction. They were "pre-GPS" navigation.
I have marked trees from the USC&GS survey done back in the 1960s. Where the set their boundary markers, they would blaze these trees apparently to make it easier to locate the actual set marker. I was told that they also blazed some trees along the boundary, but those trees are no more. But I have several remaining, one at a marker, and one along the boundary between my property and federal property.