"Blow your mind" is an English idiom that means that something has had a profound effect on someone's brain. This phrase takes its meaning from its exaggeration, since something that literally blew one's mind would kill the person in question. In a positive sense, "blow your mind" generally means that someone has been stunned or overwhelmed by something they have experienced. There can also be a negative meaning attached to the phrase if the person has been adversely affected by whatever has taken place.
It is common in the English language for people to pepper their speech with idioms. Idioms are phrases which take on a meaning that is different from the literal meaning of the words in the phrase. Through popular use in a culture, these idioms grow meanings that are instantly understood by speakers and listeners who realize that they should not be taken literally. One of the most vivid of these idioms is the phrase "blow your mind," which immediately conjures up imagery of a memorable and affecting experience.
This phrase gains its meaning from the fact that, for something to blow one's mind, it must be an unforgettable event. Such an experience may have an effect that forever alters the person who is the subject of the phrase. Indeed, the phrase is often used in conjunction with some sort of cultural event. Imagine someone saying, "You have to hear this album, because I'm telling you that it will blow your mind."
Of course, if someone were to literally have his or her mind blown, it would be a horrible event. This is perhaps how the phrase has been attached at times with a negative meaning. Most often, this occurs in the context of some sort of drug use that causes irreparable harm to someone. For example, someone might say, "All those years of doing cocaine really blew her mind and now she's in really bad shape."
"Blow your mind" is an extremely descriptive phrase when considering the literal implications of the words. The idiom can also be shortened to the term "mind-blowing" and used as an adjective. In that usage, an individual might say, "Reading that book was a mind-blowing experience; I see the issues totally differently now that I have read it."