Actually on the ridge, fingernail polish may be something you have around that should work okay. If your kids have any Goth friends, then they probably have black. Maybe Deep Purple. There is another product that would probably do the job. It is called liquid electrical tape. It is a fairly thick black rubber sealer. I use this stuff inside my wire nuts in my old shop. High humidity out there. Works great.
Amazon.com: Gardner Bender LTB-400 4-Ounce Black Liquid Electrical Tape: Home Improvement