Phosphoric acid, which is likely the key ingredient in the products you mentioned, will leave a slightly dull, pale gray finish that is fairly durable and rust resistant (certainly more so than any blue job). I've used it for other applications, swabbing the acid on and leaving it for about five minutes before rinsing and drying. Whether or not this is the process that the gunmakers use, I cannot say.
As an aside, the finish on military guns used to be a phosphate derivative. Oxpho Blue - I've always wondered if it isn't some kissing cousin to the phosphate process.
