Meaning
A large solid black square that fills the whole emoji space, the biggest of the black squares. It means black and works as a full block in grids.
It is a staple of pixel art, word-guessing result grids in dark mode and dramatic all-black posts: “Outfit check: ⬛⬛⬛ all black everything” Some use a row of them as a heavy divider or to blank out spoilers. Smaller options are ◼️, ◾ and ▪️.