Rust is an iron oxide, and as such, occurs when iron, which is an element, is mixed with oxygen. Rust occurs on steel, due to its iron content.
Copper, on the other hand, is also an element, and therefore contains no iron. Due to oxydation, copper will form a patina. A patina can protect the underlying metal from further oxydation, whereas rust will not protect the underlying metal from further rust.
The black patina on copper is indeed copper oxide, which as the other answer says protects the metal from further decay. If you see intense green color on copper, that is copper carbonate which is much more analogous to iron rust. The green copper carbonate is (and can continue to) eating away the metal, just as iron rust does.
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