This is an expression that has been around since the 1960s. During this period, people sometimes took drugs which produced hallucinatory effects; these drugs were called ‘mind-blowing drugs’. With the passage of time, any experience which affected the mind or emotions of an individual began to be called ‘mind-blowing’. Nowadays, the expression is used to mean ‘overwhelming’, ‘shocking’, or ‘astonishing’.
*Trekking through the forest all alone was a mind-blowing experience.