Bubble > Icon functions (black/charcoal)

[Bill]

Good morning everyone, my question is about the first bubble icon that
turns the screen black. What is the exact purpose of this button? Is
it's only feature to hide your screen, or is there something more to
the button? Will there be more functionality for this button?

At first it made me think of the site blackle.com, but there seems to
be no functionality beyond just blacking your screen (like you smeared
the screen with a piece of charcoal).

It would also help if a line or two about the functionality and the
way to close the charcoal were included in help.

Thanks, zBill.