Weaponblack is a common concoction in Fantasy Role-playing Games. The purpose of weaponblack is to make a sword or dagger non-reflective, which is very useful for skulking around in the dark. It is an oily substance that usually comes in a jar with enough to coat a longsword about 10 times. Once applied, the weapon will have a matte black finish, which makes the weapon almost invisible in darkness. Weaponblack usually has to be reapplied, or touched up after each battle, as it wipes off rather easily.
This substance is considered very flammable in the AD&D world, and can easily be lit on fire (weaponblack is not flammable in all games though). Several play-by-email games also have weaponblack, and have added the ability to use it on armor, but how much skulking can a person do in metal armor anyway? This is a common bit of equipment for thieves and rangers, although it is outlawed in some kingdoms.
The Complete Thieves Handbook