No need to delete, if you may use them later. Open up hypershade, select all the objects in the model, then right click on the basic lambert in hypershade. From the marking menu select assign material to selected (of something to that affect). This will give it back the basic gray lambert and leave you defined blinns for use later on.

Love the sketches. Pretty good imitation of Frank Miller's Dark Knight, but my favorite has to be the Indianna Jones riff.