No common problem. Blue smoke is indicative of oil bypassing the rings on your piston and burning up during the ignition stroke and getting exhausted during the exhaust stroke This has nothing to do with your changing the plugs or the valve cover gasket. In my opinion, it seems to me that you have worn rings on the piston in the one cylinder where you found the soaked plug. This will unfortunately require repairs to the Engine itself, ie: taking it apart and replacing the worn rings on the piston(s). Not a simple remedy...
