The easy way. Buy a small bottle of Brownell's Oxpho Blue. Degrease your small part and heat it up with a hair dryer. Pour a small amount of Oxpho Blue in a small container, enough to fully cover the part. A small degreased spray can cap works well for this. Drop the part into the cold blue and leave it for a minute. Remove it and let it dry or you can speed the drying up with the hair dryer. Polish off with some 0000 degreased steel wool. Repeat until you get the desired color. Degrease the part if it is touched with bare fingers or don't touch the part with your fingers each time you do this, it is best to do wearing thin Nitrile gloves. It only takes a few passes to make the part as dark as original, you don't have to remove the original finish on the whole part and Oxpho blue has no smell and is a durable finish. I cannot say that for some of the other cold blues on the market.