"gaelic speakers had referred colloquially to the Black Watch as the Breacan Dhu due to the dark tartan worn by them"

As far I can see I think Jim is correct. Peter Simpson a former company commander of the Black Watch and later military reseaercher on Highland Regiments writes in his book "The Independent Highland Companies" that "their sombre dress of dark cloth or dark tartan in their coats and plaids earned then the title of An Freiceadan Dubh translated as the Black Watch" He goes on to say that it differentiated them from the cotaichean or saighdearan which were the redcoats or red soldiers.

The Black Watch website gives the year of 1725 for when the original six companies were raised, a further four were raised in 1739 and were all formed into a regoment of the line.