Does anyone remember an old cartoon movie fantasy like with this boy who had a key to forbidden door that evil black goo stuff and he releases it captures his father at big ball?

The movie you are referring to is likely "The Last Unicorn" (1982). The protagonist, a young unicorn named Schmendrick, possesses a magical amulet in the form of a key that can open any door. When he uses it to open a forbidden door, he accidentally releases an evil black goo creature called the Red Bull, which captures Schmendrick's father at a grand ball.