Meaning
A solid blue square from the emoji version 12.0 colour set. It means blue and is handy for drawing water, sky and blue blocks in grids.
People use it in pixel art, colour-coded schedules and team boards, and to build simple flag designs out of squares: “Team colours sorted: 🟦 vs 🟥 this Sunday” It also works as a calm section marker in long posts. The round version is 🔵.