I have a brainbuster... I have a Cub Cadet walk-behind lawnmower. It is the SRE 621...?

Blue smoke is usually oil. Fuel smoke is more black and smells like unburned gas. A couple of things that I would do. Check the paper air filter for oil. If it has oil on it change it. Next I would put a teaspoon of fresh gas in the sparkplug hole, replace the plug and try to start it. You may have to do this a couple of times to clear out the cylinder. If it tries to run and quits after a few seconds I would suspect the carburetor. Make sure that the sparkplug is clean and free of oil. I understand from your description that you pulled the flywheel and checked the key? Did you torque the flywheel when you put it back on? It needs to be torqued to around 50-60 ft lbs. If you don't torque it enough the first time it fires it will shear the key. Have you checked the compression? It should blow your thumb off the hole when yolu crank it. Just some ideas to check. Hope some of this helps. Not too sure about your spark testing method. Even a weak spark could bring tears to your eyes. You need to make sure that it will spark through the plug.