Adding to what Rob said a couple posts up.
High Speed Steel was developed to hold its edge when near red hot cutting metal on lathes and milling machines. So if the edge gets blue the tool's heat treat is still intact. What you must not do is to dip the hot tool in cold water. That causes microscopic cracks in the edge which leads to premature wear. You have to sharpen again. What you should do when the edge blues is to set the tool aside to cool in the air on its own. It may not look pretty but won't make a difference in the edge life.
Now if you have an old tool with carbon steel, old turning tools, chisels, plane blades, etc and you blue it, the hardness is gone in the blued area. You'll have to grind back past the blue to get to hard metal again.
To avoid blueing entirely make sure you dress your grinding wheels regularly to remove dulled abrasive. It looks like the wheel is glazed over or shiny. Don't grind with worn sanding belts for the same reason or use a water cooled wheel like a Tormek.
Pete
