the acidics like vinegar, citric juices etc. will give you a brown finish. Rust finish as it may be called. Makes a nice dark brown finish, for muzzle loaders. You might try blending the blue, with the brown. Put the brown on, and then after you have washed, and dried the metal, put the blue on or vice versa, and see what you come up with, keep applying till you get the darkness or color you want. You might get a "plum" brown finish.
Wayne