This is a black middle square, its reduced version is
▪️ black small square and
◾️ black middle square, its enlarged version is
⬛️ black big square. On the Internet, people use it as a black color card, or as a decoration symbol for text content. When it appears with
◻️ white squares, we think of mosaics. Related emojis：
⚫️ black circles,
⚪️ white circles.