Blackwatch was the covert operations arm of Overwatch. It operated under the radar and beyond the reach of any red tape.
In the final days of Overwatch, Blackwatch's existence was revealed, along with accusations of criminal activities such as high-profile mission failures, corruption and mismanagement, weapons proliferation, and human rights abuses. Blackwatch soon devolved into infighting, with members taking sides as to which direction the clandestine organization would head. This ultimately led to a conflict at the Swiss Headquarters, resulting in an explosion that supposedly killed both Reyes and Jack Morrison, the commander of Overwatch and Reyes's former best friend. McCree, for his part, was disgusted with the infighting and quit before the incident took place; while Genji had already abandoned Overwatch due to personal conflicts by then. Blackwatch was officially dissolved along with Overwatch.
In Train Hopper, McCree noticed Talon's operatives using an old Blackwatch maneuver to fast-rope onto a train running half the speed of sound. This may prove Blackwatch had connections with Talon before Blackwatch's assumed dismantling, or that some Blackwatch agents are now working for Talon, such as Reaper.