A much better cold blue for sights is Brownell's Dicrophan T4.
Unlike other cold blues T4 turns steel BLACK, not blue, and it seems to be more durable then other cold blues.
Usually the reason for cold blue failure is that you don't have the steel really clean, and you can't over-clean.
I recommend giving the part a scrub with hot water and a kitchen soap.
Rinse and dry with a hair dryer (Note: Hair dryer, NOT torch or heat gun).
Then flush and scrub with Acetone and a solvent proof toothbrush...M16 brush is good.
Acetone doesn't leave any residue like even lacquer thinner does.
Let dry, then give it a good flush with 90% alcohol and dry with the hair dryer until it's fairly hot.
Apply the T4 with a Q-tip.
Allow to work, flush with water, dry, flush with alcohol and dry, apply T4.
Repeat until you have a heavy durable coating.
Rinse WELL with water, flush with alcohol, dry, apply a lube to prevent rust.
I've found that unlike most cold blues T4 won't start to pool after a couple of applications.