5.7 hemi idle and backfire

Riddler i have been looking at the two o2 sensors i took out of the car and the one that was failing has just a small amount of blackening in one area around one of the inlet holes the other one is about the same. if i understand afr correctly if the motor was running too rich the blackening would be very heavy. the moto has less than 300 miles on it and im sorta thinking the sensor may have just gone bad. what do you think?