It doesn't really affect the original bluing, but it may not match it either. Nor will it last as long, especially if you are trying to repair holster wear--it will just wear off quicker than the original. If you are trying to repair a scratch, if the metal itself is scratched, it will still be visible as a line unless you buff it out and then you will damage the surrounding finish.
I have had good luck making Oxpho last a bit longer by heating the spot to be blued with a heat gun before applying the bluing.
Another option is Dichro-Pan T4 (from Brownells). When applied to hot steel, it will give you a deep blue, almost black.