ah, you have an
excellent candidate for patching there.
the hole is in your snail's second whorl, which means it's not over any vital organs. the black you're seeing is his intestines. major organs reside in the first, largest whorl of the shell, and intestines take up the rest of the shell space.
first i want to say that it is crucial that you start feeding your snail. he may have developed the hole from rubbing up against a rock, burying himself in the gravel, etc, with a weak shell. the weak shell probably came about because you're not feeding him. although snails take in calcium from the water - the reason a high pH and hardness are important - much of their calcium intake comes from food. high calcium veggies like collard greens, kale, parsley, and spinach are great. these will need to be blanched or microwaved for a few seconds to soften them before you give them to the snail.
that said. the patching method is extremely simple. you'll need a raw egg and epoxy putty. be sure to use a putty, NOT a liquid. any putty labeled safe for use in water pipes or around food areas will work. i use a product called Aquamend. it can be found at most hardware stores - home depot, lowes, etc.
here's a link:
http://www.caswellplating.com/aids/aquamend.htm
The eggshell is good because it's an organic material that will deteriorate a little over time but won't kill your snail if it touches his body. It gives him time to heal up, plus it's basically pure calcium and might even help him to patch his hole.
Crack the egg and discard the innards. Place the empty shell in the microwave and microwave for 15 or 30 seconds, until it's completely dry. Take a few pieces of the shell and set them aside.
Fill a shallow bowl or plate with edges with dechlorinated water. You want the water to be only an inch deep.
Cut off a chunk of the epoxy putty stick. You won't need all of it - a piece just over one inch long should be fine. Put the rest back into a sealed container and save it in case more patching is required.
Knead the putty chunk until it is a uniform color. It should be tacky at that point. Flatten it in your hands until it is a thin sheet about twice the size of the hole. Covering more shell is better than less. Set it aside.
Remove your snail from the tank and place him on a wet towel. Poke or blow on him gently so he closes up into his shell. He will be easier to handle this way.
Take your eggshell pieces and place them over the hole. They should not be touching the snail's intestines or body - they should cover the hole completely. If they stick out a little and don't quite conform to the snail's shape, that's okay.
Now lay your epoxy piece over the eggshell pieces and smooth it over the snail's shell. You can wrap it around the spire and smooth the edges so they blend into the shell. Be sure it adheres well.
Set the snail in the bowl of water and wait for the epoxy to cure. Be sure that if he opens up, his trapdoor doesn't touch the epoxy. As soon as the epoxy is cured, you can put him back in the tank. Keep the water clean and feed him lots of veggies. He should be fine!
Good luck and keep us updated.
