It's you, not Tom, who is/are to blame for what happened.
The model answer here is "is". When I search for the reason, I come across a rule as follows.
If your sentence compounds a positive and a negative subject and one is plural, the other singular, the verb should agree with the positive subject.
The department members but not the chair have decided not to teach on Valentine's Day.
Then I am totally confused. Why does this rule contradict the model?