When you get the black screen do you get the DSI error occured or a code dump or id it just a black screen?
I installed the latesst rev20 on IOS 249 as I run games of HDD as I have one of the newer Wiis which wont play backup games from DVD.
This is what I did if it is any help to you (I kept getting code dump):
My USB loader used IOS 249 and kept giving a black screen with code dump text.
So i used any title deleter to delete IOS 249.
I then installed the Offcial Nintendo IOS 249 Stub.
Then (because I had priiloader blocking firmware updates) I went on system upgrade in the wii menu - this failed (progress bar stayed low for ages then reported an error) this was in the hope it may try to fix some broken things - it did appear to but I don't know if it actually does.
Next I installed my cIOS rev 20 so I could play Black Ops from HDD as Black Ops requires this rev due to the new updates. I'm sure you could just install whatever other cIOS you wanted here on whatever IOS you are using - obviously don't use any title deleter to delete IOS 349 if that is not the one you are using.

Hope some of this may help.