I've had good luck darkening brass by first breaking the surface with 600 grit emory paper then , depending on the size of the part, using a Birchwood Casey superblue pen or a cotton cleaning patch with liquid super blue on it. Wipe with oily cloth when desired darkness is reached, the oil should stop the process. Also, using the gunk from cleaning your bore, b.p. residue and such will retain the brass color but knock down the shine and darken it. It takes longer though.