It's been hot-blued, just like a gun barrel.
Quick lesson in Damascus:
Two different steels are used, each one would work quite well, alone, as a knife blade.
Being welded up with another one of similar characteristics does not necessarily make it work better.
But the end result should work as well as either of the two components.
One of the components - in this case the 15N20 - has about 2-3% nickel as part of the alloy.
When all is done and the blade is hardened and tempered, the blade is etched in an acid that literally eats away at the steel. But!! the steel with the 2-3% nickel is impervious to the acid and that acid only eats away at the other one, thus, revealing the pattern.
Now, since the acid literally ate away the vulnerable steel, it actually creates 'highs and lows' on the surface of the blade.
After I hot-blue the blade, it comes out completely black. Then I take 3000 grit sandpaper and with a hard backing, I sand the bluing OFF of the 15N20, since it is "standing" up taller than the 1095.
This, then gives the really strong visual differentiation between the two steels.
Did I make sense?