I know there is a bug, but I never found the problem. some users had lot of black banners even though they were good and present.
I didn't have report about that bug for few months, so I suppose it's fixed, it was probably a memory leak issue, that's the hardest to fix. it was a problem for lot of games, not only for emuNAND so I don't think it's your problem but I think I had to mention it, just in case.
if you have only these games with black banners, you can try to force a redump.
go to individual game settings, delete menu, delete banner animation. reboot the loader and it'll re-extract it from the emuNAND.
if it's still black, maybe the banner is not good (usually, when having black banners the game doesn't work either).
sorry, I don't have other solution, only two possibilities :
- corrupted banner, redump it (delete the .bnr file)
- bad wad file, try another wad.