Wikipedia
A darkcutter or dark cutter is a carcass of beef that has been subjected to undue stress before slaughter, and is dark in color. Sometimes referred to as dark cutting beef, they have a dark color which makes the meat appear less fresh, making them undesirable to consumers. Darkcutters fetch a lower price than otherwise ordinary beef on the market. A darkcutter's eating quality is actually better because of the ability of the proteins to remain juicy.
Stress ante mortem causes a depletion of glycogen stores in the liver and muscles, so the glucose normally used post mortem to produce ATP anaerobically, resulting in lactic acid production before the development of rigor mortis, is not available. The muscle pH stays high (above 6.0), resulting in higher water-holding capacity (sticky protein) and more translucent muscle, which looks darker because light travels deeper into the muscle before being refracted.