1. Does it add any comfort for the ball?: Yes, it makes the cage feel more secure to the snake, it works on the same principals that good hides do.
2. Have you done it?: Yes, I do it to every glass tank that I use, and my custom reptile cages are already blacked out from the manufacture. (Animal Plastics)
3. How to do it?: There are a few ways to do it, the easist is to tape paper to the outside of the cage (never use tape inside the cage). Any paper will do as long as it makes 3 sides of the tank opaque.
4. Is it worth it?: Absolutely, it costs only pennies to do and really does make a big difference to the BP.