Famous for a series of flops after he left the show what MASH actor had wanted to play Hawkeye but instead played Col Henry Blake?

The actor you're thinking of is McLean Stevenson, who played Colonel Henry Blake on *M*A*S*H*.

He did indeed want to play Hawkeye Pierce, but the role went to Alan Alda. After leaving *M*A*S*H* in 1975, Stevenson starred in a number of television series that weren't very successful.