While chrome itself cannot be darkened, it is possible to apply a tint over the top of the chrome to darken the appearance of the metal. Accomplish this using a chrome blackout spray paint kit available from your local automotive parts store. This product is offered by Dupli-Color and involves a two-step paint process.
Apply the wax and grease remover to a clean rag, keeping another clean rag dry. Wipe the chrome to be darkened with the wet rag, making certain to completely clean the entire surface. It is very important to remove any road tar or other debris from the chrome prior to painting it.
Dry the chrome using the dry rag. You will notice a slight haze form on the chrome. This is the solvent from the wax and grease remover dissipating. It will buff away after only one or two passes with the dry rag.
Apply moderately thin, even coats of the first stage of the Shadow paint process. The paint should flow but not run. Proper flow is achieved by working slowly and patiently, making certain to apply the paint in all the tight spots of the chrome. This paint gets progressively darker the more coats of paint you apply to the chrome, so you have to be careful how many coats you apply. Each coat requires 30 minutes to dry before applying the next coat.
Apply the final clear protective coat after the final paint coat has been allowed to dry for at least 30 minutes. First apply a light coat to the paint, and then follow this with at least two heavier coats to obtain the necessary gloss on the chrome finish. Allow 30 minutes between coats of clear protective coat, and then allow the clear coat to dry for at least four hours prior to handling the painted chrome.