Update- it appears to be any block. I broke a cobble stone block with one on top so I destroyed the bottom and made a floating block and I can snap into it and start drowning. Worked for cobble stone and iron just to verify it wasn't one block. It could probably kill me given the right circumstances. You could definitely make a trap with this though.
M trying to upload a photo but it's not letting me
Can concur.
Can confirm. Confused me had this morning