Sorry I didn't answer sooner. I see you could install the cIOS.
Thanks for the reply! I will try that now, and edit it if it works or not.
Edit: Trying for ModMii, it says in the credits v3.0 IOS249 (Rev 21008) (d2x v8final base56) and the game I tried to play (Dood's Big Adventure) still does not work, with the same black screen. Should I try the d2x installer?
based on that info, you correctly updated the cIOS.
d2x v8 final, based on IOS56.
Some games might require an IOS57 too, if you followed modmii tutorial, you probably have it installed in slot 250. So you can try to change the loader setttings to use 250 instead of 249 :
Settings > Loader settings > Loader's IOS : 250
Settings > Loader settings > game's IOS : 250
You also said it's "3.0", you don't have the revision number, like "r1260"?
you probably have an old version of the loader, you can find the current latest release here :
http://www.mediafire.com/download/33llk7a17ucr25f/R1260.7z
extract it to your SD or USB (the place where you put the homebrew) and launch it from the homebrew channel.
I'm gonna swap some stuff. Maybe gonna use the SD card as the game loader and the USB drive as the apps loader, will that work? (Since right now the SD card is apps loader and is 8GB, the USB drive is the game loader and is 4GB, I never thought of this before.)
USBLoaderGX can only load games from USB, but you can put the application on SD or USB, it has no compatibility differences. You only need a FAT32 partition.
which trickery ?
did you modify the game's file ? or just tried different settings in the loader?
Did you dump the game yourself? are you sure the game is working fine and not corrupted? (you can use WiiBackupManager to verify the game : option > Sha-1)