Per Jon Danniken:
I used a 6" bench grinder, and no blue either.

My blades are time consuming to remove/replace so I've gone over to a
little attachment to my Dremel tool: turn the mower on it's side, run
the Dremel... and it's done.

But next time I am going to finish off with a light pass perpendicular
to the edge to give it that flat area that Clare mentioned.
--
Pete Cresswell