are you looking to cold blue something or marking blue?What blueing are people using
Bin having a look on tube birch wood casey perma or super i think seemed faverate ? I'll have to try one eh
Edit...
cold blue i see upon a google.
i have some gun blue here, cant remember the brand tbh but its kinda hobsons choice.
i warm the part up a bit before i apply it, that is after its well cleaned.
a couple of coats should give good results but the tube is the right place to look and see how others get on, there are little tricks to pick up there too.