If you want a "good enough" option and don't mind if the finish isn't a perfect match they try some cold blue. Cheap as chips and there's some videos on YT on how to do repeated applications to deepen and darken the color along with hot water washes to "set" the treatment.
From my own experience I can heartily recommend G96's cold blue creme. I've gotten darn good results that are not far off the deep black colour of a proper bluing. But without the slight blue cast that comes from a proper old school bluing job. But you won't get that again anyway.
If you opt for the G96 creme I've gotten my best results by degreasing the metal then washing it in hot water and dish detergent. Hot water rinse then shake off the excess and apply the bluing creme with a cut down short bristle acid brush. Or an old clean toothbrush would work well too. To get into the chambers you'll likely want to use a cleaning rod and a nylon bristle brush. I'm not sure if a bronze brush would be OK or not. Best not to try it though. Rub in well, let sit for about 10 seconds so it can finish any harder to "stain" spots then rinse and scrub with the same brush in running hot water. You'll probably want to wear good gloves to avoid being scalded.
Then set aside to dry while warm. You can even use a hair dryer. Once dry and while still warm inspect and see if you want a second application. If so apply, wait 10 seconds to finish the job then brush it clean in running hot water again.
When freshly dried the black from this creme finish has a dull matt look. But once you're happy and rub some oil into it the finish takes on the proper low lustre sheen of the rest of the gun.
Last I checked the little bottle was just over $10. That and a nylon bore brush of the right size and you're ready to go.
Cole blues are not as tough as proper hot blue. And neither is as tough as a good Cerakote job. But I've found that with any reasonable care that the cold blue done this way can be very serviceable and I really don't find that in real terms it is any less durable than a not blue job. Both are pretty scratch and scuff prone after all.
