I had the same problem with only a few selected default FSX AI aircraft that would be black, both on the ground and flying and it really baffled me. That is until I read the posts over here, which were most helpful. I went into the respective aircraft.cfg files and noticed an entry "OFF.by SIMSTARTER" or something like that next to the texture= entry.
I recalled messing around with a program called SIMstarter to view/manage some of my liveries and discovered that I had unwittingly disabled a few "undesirable" liveries a while back which invariably also disabled the textures for the AI aircraft. So, in my case it was as simple as activating it again in SIMstarter.
Thanks for the great help ailchim and others!