The Rannoch Black tartan (No. 7047 in the ITI) is a fashion tartan, not a district tartan for the region. It was woven by Pringles of Inverness sometime before 1985. The STA notes indicate that they have a woven sample on file, but the sample is not large enough to allow them to extrapolate a thread count, which is why there is no graphic of the tartan available.

I imagine, however, that it might be similar to the Rannoch Red tartan (No. 5438), which was also a fashion tartan woven by Pringles prior to 1985.

If you are looking for a district tartan in the strict sense, though, neither of these would apply.
~M