Blades are pretty soft. You could probably bend it back with a big hammer and no heat, and not have any problems. Hardly worth the trouble though when you look at what they cost. Especially for a push mower, which that appears to be off of.
As far as the spark test, I'd guess that thats medium carbon. Might make a knife, probably a nice tough chopper, although not necessarily that best for edge holding. Thing you have to watch though is thats pretty thin, so you have to be careful you don't decarb it too bad working with it.
The spark test is pretty simple. It takes alot of energy to rip/break a peice out of the steel. So much energy in fact, that the little peice you do break out reaches kindling temperature and burns. If carbon is present, the carbon ignites and cause the little split/fork. The more carbon, the more forks you have. High carbon will throw a really fuzzy spark that splits several times.The spark trail is usually kind of short too, because they keep breaking up and burning. Low carbon will throw a dull spark that stays in one peice and generally travels farther.
The best way to learn how to gauge sparks is to get a couple different hunks of steel that you know the carbon content, and use them for reference. Get a hunk of cheap hot rolled welding grade steel, and a peice of 1095 or O1 and compare the two. If you can find a peice of 1050 or something midway between that would be good too.
