What do you call someone who always puts blame on others?
Blameshifter fits the bill.
blameshifting
Part of Speech: n
Definition: the act of transferring responsibility for an error or problem to another; also written blame shifting
Dictionary.com's 21st Century Lexicon
a finger-pointer - someone quick to divert attention or blame to someone else
defensive (though that's more general than just blaming others)