Just speculation, but your brass blades have not been clearcoated, and they are tarnishing. With what you described, is that once uncoated brass blades are exposed to the elements, (rain, air temp extremes ie.hot, cold, condensation) your blades are now tarnishing. If they are true brass blades, all you have to do is get some car wax and polish them out. By doing this you will get the luster back. However, you will have to clearcoat your blades with some type of clear epoxy, or spray clear to seal in the finish, otherwise you will be back where you started. I'm surprised they are not clearcoated. Also depends on how much you spent for the blades. Higher quality blades should not have this problem.
One other thing, you mention the word rust. If the brass blades have actual rust on them, maybe they were laying on top of something steel and the rust transferred over. If you are using the word rust as, the blades are a dull gray of getting black in color, then that is tarnish, just some more info for you.