I believe the whole myth of the black GG1 has it's roots in the factory fail Lionel 2332 paint job rather than anything the PRR did.
Bruce
The Black GG1 myth may have started back in the 1950's when Bill Vagel, a large Lionel dealer at the time (and a friend of Josh Cowan) brought five GG1s to the factory and had them repainted black and re-striped. Those GG1's have to be out there somewhere in an extensive Lionel collection.
Probably known examples exist, since Greenberg's lists a black 2332 in their price guide.
Bruce
I never take credence in anything put out by Greenberg's. Their price guides have been so far off the mark that its like fiction writing. IMO, they are a self styled authority out to sell books and nothing more. If you want good historical info the McComas Tuhoy Lionel books are well researched publications.