That's the scale that's usually present after heat treating. Sometimes folks leave it, then carefully finish grind....the blades itself looks like they tried to grind to about a 120.... but missed/left a lot of heavier grits scratches in there.
Many black smith shops sell a product that leaves that finish. I think DL Drew on another forum discussed it once.
But as others have said, you can obtain that finish from heat treat. If you check out my bushcraft machetes that’s from heat treating.
In my experience, it's really hard to nail a perfect grind on both sides and not have an errant scratch in the edge of that black finish. You have to be spot on to keep that line crisp. The thinner the stock the harder to do.