Brain's reaction to total darkness vs. blindness?

I visited Aillwee Cave yesterday (really cool cave, btw!) and the tour guide stopped at one of the deepest points of the cave to give a demonstration of total darkness. It really was total darkness, your eyes couldn’t make out a single outline or shape or anything.
As a bit of trivia the guide also told us that if one were to stay in total darkness for extended period of time, the brain would start to react by exaggerating sounds (water dripping would become conversation, etc.) and eventually create full blown hallucinations.
This sounds really fascinating – and an interesting (albeit time-consuming) alternative to drugs – but now I’m curios about the difference between being in total darkness and being blind. I’m pretty sure blind people don’t walk around with full blown hallucinations, but why does the brain behave differently in these situations?
I’ve tried googling all morning, but haven’t found anything that explains this, so I thought I’d try here…..neurologists, ophthalmologists, physicians, psychiatrists, etc…...anyone with knowledge/experience of these things: please enlighten me!