Correct! That's the way I learnt it. But Dr. Phil @Philip should be the authority here and in order to avoid his usual pomposity (kidding kidding!), he abdicated!
Actually according to the "Grammar Police", both answers are correct but technically "whom" is better but no-one says it that way.
Technically, that "whom" is correct because it's the object of the verb. Yet almost no one would say it that way. Does that mean everyone's wrong? No. It means that, when the pronoun's at the beginning of a sentence, even the most formal writing can use "who" as an object.
So the OP should have said, "Whom is to blame?" wtf?
It is somewhat of a surprise that they were added before the secondary camp is ready to go. Obviously Mountcastle is being held back for service time related issues.
I am actually not sure how long he has to be down this year in order to not accrue a full year of service time. I would assume it is pro-rated, with every day on the MLB roster this year counting for 2.7-ish days of service time since the season is 66 days long instead of 180, but I am not sure.