Back when I was a kid I played with some of those black MIDI files. I think one of the tricks is to put the same melody on multiple octaves. It makes the sound fuller without going dissonant.

There is also a phase like effect that can be done by playing sgales really quickly. I think I saw that in one of the original Touhou black midi files.

And of course there is pressng all 88 keys on the MIDI piano at the same time. I would like to know what that sounds like on a real piano though.

I think black MIDI starts as a rendering artifact in a Japanese music notation software. The name was "Frieve" or something. It renders as many notes put in a measure. Without expanding the view. That is sort of what's going on in the Wikipedia pic.