My Stool Is Black, Whats Wrong Does Anyone Know Whats Wrong With My Cat?

Does anyone know whats wrong with my cat? - my stool is black, whats wrong

my cat is a cat, black and white 15 years old. It has recently been drasticaly to lose weight, but he ate almost twice the amount that you eat normally. I deworm every month, so it is not worms. more normal stools. He also started meowing like crazy, which has continued to day, but they are meowing environment. I think your body is just down the closure, due to age.
Who knows what is wrong or how I could gain weight? I do not want him to lose a lot. : '(