Is this an old blaze or a natural occurrence? It is alongside an abandoned trail, and is at about the same height on the tree as other blazes along that trail.
To me, that looks questionable - I've always determined it by whether or not there is a matching mark on the other side of the tree. It is my understanding that typically (other than at a switchback) blazes were cut in both sides of the tree. If you see something similar on the other side of the tree, chances are better it is a blaze.
It might be, depending on how it lines up with other blazes...or it could be an old wound too.
If you can see old cut marks around it that helps ID it too.
Sometimes blazes aren't on both sides of the tree, or on the trail facing side. It's pretty random. I've seen 3 dots on the Skyline Trail, and only 1 dot on the really old ones.
I'd vote for a blaze - a horizontal rectangle which makes it look man made. The chance that a branch or tree top happened to hit just horizontal is very little. Of course one never knows!