The origin of Popeye's superpowers: a mad scientist working for Navy attempts to make super-seamen, which results in a severe, strength-sapping iron deficiency cured only by spinach. That plus the severe physical deformities inherent in the process made them shelve the project. Popeye has been unsuccessfully trying to sue the government ever since.
Bluto was also in the same experiment, after Popeye, so his version of the formula was without some of the flaws of the original formula, but didn't work anywhere near as well; it also made Bluto far more prone to rage and violence than he had previously been.
Both versions of the serum also caused the subjects to develop changes in their perception of other human beings. Both Bluto and Popeye thought Olive Oyl (diagnosed with anorexia nervosa) was actually a smokin-hot babe with sexy curves, and no one had the heart or nerve to tell them otherwise.
Unfortunetely, in the case of Popeye, the serum also caused genetic malformations that make his body produce a virus that infects close relatives like his father and newphews with the same physical deformities.