As you know you can't alter the exposure already given to the negatives during the development stage. You will find when you print these negatives it will be similar to printing very underexposed negatives. The typical under exposed negative, when printed so the middle values are correct, will have no blacks. Likewise when printing your overexposed negatives for a correct middle value there will be no whites. The whites will be gray. Similarly if you print the whites as white, they middle values will be way too light.