long, black, leather is correct.
Adjectives are always in the following order: opinion, size, age, shape, color, origin, material, purpose.
No one really knows WHY we do it this way. Most people don't even realize they're doing it and if you asked them if there was a rule, they'd say "no". I don't think it's even technically a "rule". But everyone does it this way and if you don't, it will sound weird
The only real exception is when one of the adjectives is actually part of the name of the object, as in a Great Dane.
A green Great Dane and a great Green Dane are two very different things. One is a jealous dog and the other is a really good politician from Denmark who cares about the environment