See inside A "say () for () secs" block where the second slot is a dark round slot that accepts booleans.
I found it at https://scratch.mit.edu/projects/67942410/ https://scratch.mit.edu/projects/167366522/ used to contain a "ghost block" (an outline as if a block was running of a turn block with no fill) in 2.0, it makes me wonder what other weird blocks have glitched into existence. I found something similar at https://scratch.mit.edu/projects/278950351/, along with what appears to be a floating input.
