If it is effecting one bank only, it could be a vacuum leak on the intake on that side. Very doubtful that all the injectors on one side would go bad all of a sudden. If the O2 sensor on that side is showing lean, then that means the engine will be dumping fuel in to compensate for what the O2 sensor is reading. Which would cause what you are describing. Remember, the computer tries to compensate for what the O2 sensors are telling it. Which means there has to be a vacuum leak on that side that the O2 sensor is picking up as a lean condition, so it is dumping fuel in trying to compensate, thus the black carboned plugs, and poor idle quality. Under hard acceleration, you wouldn't be able to tell as bad because the computer goes to full rich under wide open throttle anyways. There is an old mechanic trick to finding a vacuum leak. Get a small propane torch for like brazing plumbing pipes, make a way to attach a small vacuum line to the nozzle. Open the valve on the bottle, but DON'T LIGHT IT. Using the vacuum line to direct the propane, lightly spray it around the intake, vacuum lines etc. Try to concentrate on small areas at a time. If there is a vacuum leak, it will suck in the propane(won't harm the engine) and you will hear the RPMS raise up for a second.