"The Black Watch regiment was formed not many years after the union of 1707 as an elite regiment known colloqualy as "The Breakan Dhu""

Highland Companies were raised by the Scottish gvt in the 17thC. So although they (ie the Watch) were made an official regiment of the British army shortly after the union of 1707 they were in existence prior to that. According to Peter Simpson (a former company commander in the regiment) in his book "The Highland Companies 1603-1760" the name Black Watch (Am Freiceadan Dubh) was probably first used in the 1670s.