Well, the quality of steel in lawnmower blades is kinda like adult diapers, it depends.
In my experience, older blades are made of hardenable steel. New blades, especially those from the big box stores, are made of low carbon steel, and really aren't worth working with.
That said, I was recently told that the blades for high end mowers are still made of medium carbon steels.
IMHO, a spark test would be a good place to start, then harden a piece and break it. If it does, indeed, break, then it's made of something you can work with.
God bless