IIRC, "Default settings" only resets the settings under the settings menu, not the individual game's settings.
So, if you changed any individual settings for the game, they were not reset. You have to go to the games individual settings and select "Default gamesettings"
(When I say "individual game's settings," I'm talking about the settings menu you can select when you select a game. A screen pops up with a picture of the disc along with the options "Settings" and "Back".)
Also, if you're using a USB Loader GX build from the google code's BETA discussion, there is a known problem caused by the use of devkitPPC r24 that causes a random black screen when you launch a game.
Please go to the beta discussion again and download the latest build revision.
(When you update USB Loader GX, it's a good idea to delete ALL your settings files (.cfg) before updating.)