It is completely acceptable that a person can do something wrong from time to time in their lives - there is nothing wrong about that. That is why you and your partner should be capable of accepting blame when that is the right thing to do (you shouldn't be forced into accepting blame or anything like that). If one person in a relationship can't accept blame for anything, that means that that person can't grow in the relationship.
Accepting blame is an important part in personal development, as that is one of the few mechanisms people have to learn right from wrong and to be able to improve their behavior when it comes to socializing with others. A person that cannot accept blame is a person that cannot be taught how to improve their behavior.