I had a similar problem yesterday, I followed all the steps of the guide, up until installing Aroma, then I booted the console to make sure everything was working so far. The next step was booting without an SD card inserted, supposedly the console should boot into a black screen before doing this step, but mine did manage to boot to the main menu, it just didn't load minute or Aroma. So I finished this step and confirmed that the console now loads minute and the plugins without the SD card inserted, but now when I try booting up the console with the SD card inserted I get the same issue you're having: the Wii U logo shows up for a couple seconds, then a black screen. After double checking the guide I went to Step 3 on installing Aroma, which tells you to download a root.rpx file and put it in your wiiu folder, I already had a root.rpx file there before doing this step, so I tried using my file instead and now everything works as intended. Not sure if my issue is related to yours, but you can try booting with the SD card inserted using this root.rpx file and see if it works.
