When someone blows your mind it means that they have impressed you by doing something that you thought was impossible.
Something can also blow your mind by you seeing something impressive or amazing.
The blow your mind saying is to affect you strongly by something and it's usually because something or someone has impressed you.
Some examples of sentences that can be used to say blow your mind or ways that something can blow your mind are.
The cake simply blew my mind, it was so good that we ordered 3 more of them!
You have blown my mind with such a mesmerizing performance.
She will blow your mind when you have tasted her cooking.
I have received the most mind blowing news ever.