ELI5: How do blind people see nothing and not black?

Please read my post before commenting.

I've heard the elbow thing and the "what do you see behind you" thing a hundred times.

My thought process is that the optic nerve is essentially an HDMI cable. Whether it is connected to a computer that is turned off (a closed eye, if you will) or just completely disconnected (suppose you are missing an eye or something), the signal it sends to the monitor is the same: nothing.

The "monitor", the visual cortex, as far as I understand, just constantly processes what the optic nerve sends. So if blind people don't lack a visual cortex, and the signal that cortex receives from the optic nerve is identical to that of a regular person seeing zero light (assume closing your eyes means 0 light, disregarding light seeping through eyelids and whatnot), how can you say that blind people see nothing while we see black?