I also believe that Black Watch should be restricted to Black Watch soldiers and family of the Black Watch.

"Ancient Campbell is the same as Black Watch tartan--hardly surprising, since that illustrious regiment is a Campbell regiment, raised by the Duke of Argyll in 1739. (Indeed, as Alastair Campbell of Airds Yr., Chief Executive of Clan Campbell, has indicated in his authoritative book Campbell Tartan, the Black Watch tartan may well have been adopted as the clan's tartan because so many members of the clan were already wearing it, owing to their service in the regiment.)"

This was taken from the Clan Campbell Society of America's web page
Just thought I would throw in my two cents for what it's worth.