I assume they use some cold blue on it.I think I had mine done when they had a special.
I didn’t have my slide refinished, I just had them seal the part they milled with some type of whatever chem they use. You cannot see any bare metal.
This is who I used and I’ll be sending another soon. He starts at $75 for a cut. Price goes up if you want cerakote, taller sights, etc.