Many times the cylinder that shows it to be running rich is not the one that is causing the trouble. It is usually the cylinder(s) that shows the lean condition that has the problem. Lets say the injector in the 2nd cylinder is partially plugged. The oxygen sensor will notice the lack of fuel and will try to correct the problem and supply more fuel. This may get the fuel mixture in the 2nd cylnder closer to the correct air fuel mixture, but in the process will make the other cylinders run rich with fuel and would show up as dark or black sparkplug porcelains. Can you have a cylinder balance run on the engine to check out the power differences between the cylinders? This would point to the troublesome cylinder/injector.
Happy New Year!!!!
John