Even by purists and tartan snobs, the Royal Stewart tartan and its variations (including the Stewart Black) are considered generic tartans anyone may wear.
Point of Clarification: The Royal Stuart tartan is an de facto "open" or "generic" tartan. Technically, the Royal Stuart is a "Royal" tartan, and is worn with the permission of the monarch by pipers in several Scottish regiments, including the Royal Scots Dragoon Guards, the King's Own Scottish Borderers, the Royal Scots and the Royal Highland Regiment (The Black Watch).

From the RSDG's web site:

The late King George VI took a great interest in the Pipes and Drums, granting them the privilege of wearing the Royal Stuart tartan
From the MOD's Royal Scots web site:

The Regimental tartan is Hunting Stuart (left, and stylised in the page margin) which is worn by all ranks except Pipers who wear the Sovereign's personal tartan, Royal Stuart (right); an honour granted by King George V to mark the tercentenary of the Regiment in 1933.
Finally, from the STA's web site:

Tradition has it that those who have no tartan of their own can wear the Black Watch (The Universal or Government Tartan) or the Hunting Stewart, but not the Royal Stewart without the express authority of the Queen. However, commercialisation in recent times has rather blurred this. The one tartan which cannot be worn by anyone unless the Queen's permission has been granted is the Balmoral.
Now, before I upset anyone who wears Royal Stuart, I am, by no means advocating that it be a "restricted" tartan and can no longer be worn by anyone -- on the contrary. But, following the mantra of "Know your tartan", I felt that it should be explained that at one time, it was not an "open" tartan, just as the Government sett (the official name of the Black Watch) was not open either. Times have changed, and tartan evolves, just like the kilt has. But, one must know where they have come from to know where they are going, or as the Lakota say, "A people without history is like the wind in the Buffalo grass."

Being a reference librarian, I just I had to supply some background information on the Royal Stuart. :mrgreen:

Slainte!

Todd