I'll add, though, that Oxpho-blue is a phosphate based colorant, sort of like thin Parkerizing. Really almost dark charcoal gray more than blue. Despite the lack of color depth, it is less likely to initiate after-rust than others I've messed with. Van's is another one like that.
These thin coloring layers really don't wear well, IME, either. If you want something more permanent that may be done at home, look into rust bluing.
