A Black Rifle is one of the most fearsome weapons ever designed by the hands of man. It was designed for shooting large herds of roaming school children in one session. It can penetrate even the most heavily armored American army tank; bring down a satellite in orbit a hundred miles above the earth; blow all four legs clear off an elephant; sink a battleship; and destroy the soul and morals of even the best of us. They are best left alone as the diabolical creatures they are. Don't give one of them the chance to enslave you.
Or, if you're NOT a liberal, it's a generic term for a military style rifle that typically shoots a small caliber center fire rifle round. They are often black in color due to anodizing / phosphate coating / paint, to minimize the amount of maintenance required to them.