Your mind exploded when you heard this. Blew is used as in blowing up. An explosion that was created from learning the news. Upon hearing this news, it was so amazing, your mind blew up like it was wired to a bomb.

"blew" in the normal usage means the past tense of the verb "to blow" which means to create an air current or to violently move. when you use that word in the phrase "that blew my mind" the word "blew" actually is a metaphor which suggests that something was so incredible it caused your mind to explode.