For something found locally, the Birchwood - Casey Super Blue creme is easy to use and will provide color. Oxpho Blue if you want to order something.
Make sure you thoroughly clean and degrease the area to be cold blued. It helps to use a hair drier or heat gun to warm the metal, but do not get it really hot. The use of heat is simply to drive away moisture trapped on the surface of the metal. You can also use a propane torch, sweeping the flame across the area until you no longer see the moisture evaporating from the surface. Remember, you don't want the metal real hot, you just want to rid it of moisture.