If you have a matte finish, scraping will ruin it. Razor blade will ruin it. If it is matte on a Beretta Onyx, it might be their Bruniton finish, which (I think) is actually a film, and should not rust unless the film itself has been damaged. I'd try the least aggressive approach that I could get away with, which for me means starting with light oil and a bit of stainless steel scrubbing pad from the hardware store. If it's a high polish blue, any of the techniques mentioned will work.