A product I'm pleased with and use is "Forever Black". It's dye made for auto trim that came recommended from a car detailer buddy. A 4.0oz bottle will run about 8 or 9 bucks. I bought it online a couple of seasons ago for my Ultra and still have 3/4's left. I use it about twice a year. Details are here:
Forever Black products will beautify and protect tires, moldings, trim, and bumpers.
I've also been seeing the use of peanut butter on the car detailer websites. Apparently they are smearing it on and wipe it off. Seems pretty eco-friendly, and also temporary to me. It's probably the peanut oil that blackens the trim (I've also heard that baby oil will work, so peanut butter doesn't surprise me).