The expression is actually, "This IS going to blow your mind." It is the same as saying, "You are going to find this so amazing!"
It's not a rude thing to say, but you usually only say it around friends.
Salesmen usually say it to make it sound like their products are the latest invention that will surprise and awe you.
I usually say it to girls on the second or third date.
People's minds rarely get blown.