Hi,
Too much of even a good food can be toxic. With something like pomegranate, which is high in nutrients and sugar, you would only want to offer a few small seeds per day depending on the size of the bird. And probably not on a daily basis. If you have fed too much, it may be having a toxic effect. The best thing to do is take him to an Avian Vet as soon as possible. Black droppings can be due to internal bleeding, but can also mean something else. Anytime a bird changes their eating habits or behavior this way it is best to take him to a vet.
Take care,
Brenda