I really like using Oxpho Blue
Even when bluing a small piece like the Loading Beak (on the UpLULA); you can get almost any depth of color you want
From a mock parkerizing, to a deep "Colt" Blue
The more you burnish the piece, the deeper and richer the color
The trick is; after your last application, where you've gotten to the color you're after, rinse the whole piece with clean Oil
This helps set the final color