I am no expert, so take my answer with that in mind. I first clean the blade in petrol or acetone. Then I lay it on a papertowel on a plate with dept enough to keep the blade under surface when soaked. Then I ad Oxalic acid powder and boiling water. This will make the steel dark gray with a matte surface. I do my “washing” in a plastic jar with chards from a coffe cup and some WD-40. After that I does a light polish with soapy steel wool.
To anwer you first question, I don’t think the acid will penetrate the black surface (since I need to remove any fat with petrol to make a even dark surface). To answer your second question, what look you like is your choice, but it can be done to any surface as long as it is cleaned with petrol or acetone first. The acid gives a dark gray matte surface that can be polished with steel wool to a semi matte finish. The less reflective you want it, the darker or matte you will keep it. With my method, carbon steel will be closer to black while stainless will be dark gray.
