Are there that much differences between loaders in term of compatibility anyway ? I thought that it was all bound to the cIOS loaded when the game starts, I mean, the loader only starts the game then is wipped out of memory, right ? Never had compatibility or stability issues with GX so far and it supports the last cIOS version so it's good to me.
To OP, black screen means IOS issue. Depending on the tutorial you followed, you should have several cIOS installed to different slots (223,224, 249, 246, etc). Some games are only compatible with cIOS that use a specific IOS as base (in the case of Black Ops, IOS57) and you need to configure the loader so it uses the specific cIOS slot for that specific game. Off course, if you setup the loader to use an invalid IOS (stub or non-existant), it won't work at all.
Problem is that if you don't tell us which tutorial you followed, what you did install and what is your cIOS setup (i.e what cIOS with which base is installed to which slot), there is no way you can get a straight solution. Many people will pop up and say "I got that issues too, I used loader XXX and it worked" or "Install CIOS 248 and it will work" but the truth is that it all depends on YOUR setup.
I agree a simpler solution would be to "mod" everything again but it could save you some hassle if you knew what was installed in the first place on your Wii