I just wrote "Paper Only" on the side of my rotary cutting blade that has "dulled". I have two holders, so one has the new blade and now I can identify my paper cutting one easily. I used a Sharpie. I have done this with the scissors I use for paper only.
I join in on the question "what rotary cutter sharpener works?".
I'd like to add that I store dull blades in, the small jar that, "imitation" soup bullion comes in. It's a perfect fit for the 45mm blades and it has a screw on cap. too.