When I create a new document in Adobe Illustrator, a dark grey block appears around my workspace and I do not know how to disable it.
What is this and can I turn it off?
This is actually the usable workspace.
Apparently, you are at a very low zoom level. Although it's very big, Illustrator's workspace isn't of infinite size. The area beyond it can't be used to work on.
In your example, the white area is your current artboard; the dark grey area is usable workspace and the light grey is, well, not.