By ,"Black Irish" do you mean of native Irish Celtic origin.? It is not a term we Irishmen use but rather a dreadful Americanism to describe those who emigrated to America after the famine in contrast to those of Ulster Scots origin. Please don't use it anymore than you would say,"nigger", or other derogatory ethnic terms.
Cowan can be of Scottish or native Irish origin.Cowan is common through the Scottish Lowlands, especially Ayrshire and Dumfriesshire.Many Colquhouns (Cahoons) there took the name Cowan. The Cowans of Corstoun in Fifeshire were the proncipal family in that part. Cowans were also found in Kilchoan (Kill Cowan - Comgan's church). Cowan in Scotland is derived from St.Comgan. In Argyllshire Cowan is a shortened version of MacGilleChomhgain or "the son of the servant of St.Comgan. The Clans Dougall, Colquhoun and Donald had septs named Cowan and MacCowan.
The Irish name Cowan is derived from O Comhdhain and likely has the same ultimate origin as Cowan in Scotland. Cowan in Ulster is used as a synonym of MacCoan, Coyne and MacKeown and sometimes it is an abbreviated form of McIlhone.
All that one can say is that the name is derived from an early Irish saint and without careful genealogical study it is difficult to know if a particular Cowan derived his name via Scotland or directly from Ireland.Since the name Scotland is derived from an early Ulster Celtic tribe in the northeast one could assume the name was taken to Scotland from Ireland and came back with the Scottish plantation of Ulster from the Lowlands.