The phrase "black rectangle" typically refers to a shape that is black in color and has four sides, resembling a rectangle. It is often used in art, design, or technology to describe a simple, solid black shape. Full definition
I was going to do that last paint step on the middle column's black rectangle with white paint in the center but ended up liking it as is. (diyshowoff.com)
We fixed an issue where the solid black rectangle of the keyboard could sometimes be seen for a second after entering your pin on the Lock screen to unlock your device. (blogs.windows.com)
Now take your iron on transfer paper and print out a big black rectangle large enough to fit your traced head in. (theinspiredroom.net)