Blackwatch was the covert operations arm of Overwatch. It operated under the radar and beyond the reach of any red tape.
Blackwatch's existence was kept a secret from the public, but there were always rumors of Overwatch carrying out black ops missions. As Overwatch's influence waned, people within Blackwatch sought to bring down its parent organization and turn it to their own ends. In Overwatch's final years, two decades after the Omnic Crisis, Blackwatch was revealed, along with allegations of corruption, weapons proliferation, assassination, coercion, kidnapping, and human rights abuses such as torture. Governments called on the United Nations to put an end to the "aggressive and repeated violations of many countries' sovereignty." This led a special UN committee to launch a lengthy and secretive investigation, which eventually led to the dismantlement of Overwatch, and therefore Blackwatch.
Years afterward, McCree noted that tactics used by Talon seemed to be based on those used by Blackwatch, and one of the Talon soldiers recognized McCree, implying a connection between the two organizations.