"It" would be any event or sensory input. "Blew" in the sense that a fuse is blown — rendered incapable of carrying any further current. A mind-blowing experience is so overwhelming that one cannot take in any more data. The circuits are scrambled or melted. I believe the phrase came into existence (or certainly into popular use) in the 1960s in reference to psychedelic drug experiences. It seems generally (though not exclusively) to refer to a pleasurable, though possibly debilitating, experience. I was in college in those days, but have no first-hand knowledge of this phenomenon.

I was not around the forum in your earlier days here, but welcome back!