What does it mean being referred as a "black square"?

I haven't encountered such term the whole time I was playing the game, although particular "color" scheme present in the game are resource zones. To check on this, go to the Battlefield, and from there, click on the binocular icon at the lower left of your screen. This will give you a view of the entire world's map (which you are currently in). Click the hamburger icon on the lower right part of the screen and enable the top most button (which is currently disabled by default) this will tell you which resource zone your command center is currently at. Occasionally, there are special events too that spawns special resource or bonus spots. This is most likely what you are asking. These spots tend to give the Corps that gains control of it some special bonuses.