It's quite possible the sign you're looking at is porcelainized enamel. The lettering is silk screened on a sheet of metal using special paint (lightest colors first, then the darker ones, followed by black, allowed to dry in between each color). Then this is fired at high temperatures in an oven to a porcelain finish, just like dishes. The various layers of paint in the results give a mildly engraving-like look.

Re those signs with the raised black Letters: black pigments withstand the effects of ultraviolet radiation best, so the black paint lasts longer, allowing the weather to eat a way at the surfaces which had been coated by the white paint.