Have you tried cold blue a lot of people don't like it because they don't know the trick.
The trick is HEAT
Yep just heat, you need the parts clean of any oils, or finger prints.
Then heat until it is just to hot to hold, then cover it quick with the cold blue, for small to med. parts I put in a zip-lock bag & pour the blue over it, zip, & shake to cover parts.
Rinse, reheat, & repeat until the color is dark enough for you. (normally 2 to 3 coats)
then reheat and coat with a good quality gun oil in a plastic bag, keep warm & coated for 24hrs.
You will have a good looking finish that you can be proud of.
Birchwood Casey is the type I use, it is low cost, gives a good finish, & is easy to use.
Birchwood Casey: Finishing List