It is certainly debatable, some lists include it and some don't. Baltimore is not in the traditional industrial heartland or even the Midwest, but it's economic makeup in the mid-20th century is very similar to traditional Rust Belt cities (large ports, metal production, manufacturing).

includes - https://www.thoughtco.com/rust-belt-industrial-heartland-of-...

doesn't include - https://en.wikipedia.org/wiki/Rust_Belt