What is "blackener"

Usage examples of "blackener".

He remembered the tents pitched over the concrete foundations, the standing in line for chow with mess kits in the mud, he remembered the waiting on the ready line in the fleece padded shooting jackets made from old CKC blouses, the smell of burnt cordite and the ringing ears and the carbon sight blackeners that smudged up everything and the two or three privately owned BE slopes of the top notch shooters, he remembered all of it, the heavy clinking dull glittering unexpended cartridges in the hand, the long deadly streamline disappearing of a cartridge slipped into the chamber with the thumb when you were firing singles, the swinging white spot marking off the bulls and the big red flag rising from the pits three hundred yards away.