I think i found solution for your issuse.
While i trying resolve mine, in progress from invisible outdoor wmo groups i get black ones like yours.
it could be 2 things:
1st check MOMT chunk for 0x08 (darkened material)
or if your wmo is using MOCV chunk do small experiments in hexedit with colors, change some of those 00 00 00 ff to 3b 3b 3b FF (or less) and check what is happen.
as you can see on following screens it solved my problem using cataclysm original wmo files in 1.12 wow
