Weaponblack

When rangers or thieves apply this oily paste to their weapons or armor, it makes the metallic surfaces non-reflective and nearly invisible. Modify their base chance to hide in shadows by +5%. A coat of weaponblack lasts until the character engages in melee combat, at which time enough of the substance flakes away to negate any camouflaging advantage. The substance is flammable; if lighted, a sword coated with the paste will become the equivalent of a flametongue for 2-5 rounds, and will also inflict 1d4 points of heat damage upon the wielder unless he is magically protected. A vial of weaponblack contains 1 application. This substance is uncommon and only available through shady under-the-counter dealing.

[Complete Ranger's Handbook]