It seems to loosen everything up, but I haven't gotten to really inspect them closely. The only way I know of stripping rust all rust including black that won't remove any good steel is the electrolysis method.
The blades I put in the Evaporust had a LOT of rust and pitting. I'm not sure how it would work on a blade with just a little black rust or surface rust. It would probably be better.
For $8, not a bad item to have around.