From a Wiki Source:

The 3rd Battalion's nickname "Darkhorse" sprang from the radio call sign it used during the Korean War, chosen by Colonel Robert Taplett, who as the Battalion Commander (CO) of that time had the call sign "Darkhorse Six". The name fell out of use until 2003; during the training to return to Iraq in 2004, the Battalion Commander, Lt. Col. P. J. Malay, requested use of the "Darkhorse" call sign as a homage to the battalion's bravery in Korea. The nickname stuck and the 3/5 Marines now use it on their unofficial patches.

One of Ambler Furry's Rules of Combat: "Tracers work both ways"