Wow, thanks for the fast feedback!
I just uploaded a quick patch with some fixes, thanks to you! Now, you can press 'M' anytime to show or hide the black screen, so that annoying bug won't be a problem anymore. I also added a little sound and a flash effect for when the hammer is fully charged, hopefully that makes timing a bit easier!
The resolution problem is a bit tricky. I built the game around a 16:9 screen, and it looks good on my end, but I can't be sure what happens on other types of monitors. It's definitely something I'll need to look into more.
It sounds like you were about to beat it! I'd be really happy if you gave it one more go. Thanks again!