320 grit is about right. The finer you polish, the more difficult it becomes to get an even, consistent finish with rust blue. Over-polishing is definitely a mistake. Generally, rust bluing properly done will provide a velvety matte finish that is closer to black than blue. The exact color will depend on the formulation of the steel and the amount of work you put in to the job -- usually, the more times you repeat the rust-boil-card process the deeper, more even and more durable the final result.