You really need an incubator, eggs need the correct humidity. And eggs need to be turned 3x daily unless you have an automatic turner with the incubator. If they aren't kept at 100 degree and 85% humidity, the eggs will probably die. The humidity keeps the membranes from sticking to the egg and this traps the baby duck in the egg and then it dies. Also the turning is very important, it can be done manually by putting and X on one side and 0 on the other side so you can keep track of turning them. If you dont do this the embyo will be born with deformities. I just learned all this now since I just had 5 ducklings born last night and this morning. As for the black tip, it doesn't sound good. Try to candle (using a flashlight in the dark) to see movement. In the fatter part of the egg is where you will see the air sac. Usually you can see veins or movement. I suggest doing research on the internet