To hit or attack (an opponent) from their blind side is to blindside him. The 2008 stock market crash blindsided many investors. "Blindsighted" is a misspelling. "Blindsided" is correct.
Blindsided means to attack or assault from someone's vulnerable position or where they could least expect or uninformed.
"Ann divulged that although she was blindsided by the new development, she's still happy with it."
Blindsighted is a misspelling of the actual word, blindsided. It has been utilized by several article writers, story writers and pressmen in their attempts for punning or rhyming headlines.