Try using a program for the Homebrew Channel called BootMii Config Editor, which will allow you to set the options for loading BootMii without having to load BootMii. I had a similar problem, and my default BootMii config was set to progressive. BootMii was loading, but was not visible (because if you press reset while in the black screen, it loads the System Menu, which is the first option in BootMii). It was fine after setting it to NTSC instead of progressive because the TV I was using couldn't run Progressive Scan. Not sure why BootMii's config defaulted to a mode my TV couldn't handle, but this solved my problem.
To sum up:
1) Check to see if you have the same problem I did (press reset after 10 or 15 seconds after loading BootMii, or black screen booting Wii). It should boot to the system menu; if it does, continue to the next step. If not, this is not your solution.
2) Dowload and install BootMii Config Editor (google this Homebrew app, follow instruction to install).
3) Run the BootMii config editor from the Homebrew channel.
4) Change the display setting to NTSC (or PAL for European TVs).
5) Save the config.
6) Try running BootMii from the Homebrew Channel to test if this worked for you.
Hope this helps someone . . .