iPhone 3GS water damage black screen

ichigo, the first thing that I would suggest is that you follow this guide. It was written for the 3G but is the same principal as the 3GS. Once you got it accomplished, re-evaluate the damage. It will also prevent corrosion down the line. If you do not clean your phone now, you may end up with damage in a couple of weeks. Anyhow, once you did that see what happens. Since your screen remains black, my gut feeling would be that it is short cicuited. You will most likely have to replace it. Not a terrible job to do and not really to expensive. Here is the how-to and the part is available at places like this. I prefer the whole assembly since it causes a whole lot less headache and is far more convenient. Of course if you prefer the LCD by itself is available here. For a good quality item, I suggest this place. Hope this helps, good luck.