Ok I was wrong. You can leave some of the grottos you warp to. Usually both screens will turn black, but you can still walk around and use items (you can hear the noises).
When leaving the grotto I warped to when using the one in zora's river inside of that stone circle I walked around a bit and fell into lava or something. the screen appeared for the one second again when I hit the lava. I wasn't really able to see where exactly I was, but this might have been the ganon's castle surrounding area.
I think I've tested almost every grotto now and I always just ended up in another grotto. Only exception was the standard one which warped me to lakebed. we should really try to leave all the grottos now and try to end up in a useful place.
When you end up in the area which you believe is ganon's castle area, do you think anything in specific would happen if you die?
Maybe the screen comes back, or maybe you just get a soft freeze.
If you could get your screen back by dying there this could be a major breakthrough.
Also, I wonder what would happen if you enter a loading zone when you have the blackscreen.
I have a feeling that you would get a soft freeze, but if not then that would also help alot.
Here is my new water temple small key clip:
http://www.youtube.com/watch?v=SRjta0gpdFg