Brownells still carries it, but it is a nitric acid and selenic acid mix, according to the saftey sheet, and those are also the ingredients in 44-40 blue, IIRC. The phosphoric acid based Oxpho Blue is better on corrosion resistance, but it's not as black, and if it doesn't work on silver solder, I'd forgotten that. With 44-40 you get darker color, but I have had after rust with it and other non-phosphoric acid-based cold blue. The solution to that is I spray a highly penetrating and alkaline cleaner on the blued spot to penetrate and neutralize any acid traces (I use Formula 409, but there are others), rinse that off in boiling water so the heat self-dries it, and then I apply LPS-2 which penetrates and adds rust inhibitors.