Spock, from the planet Vulcan, has green blood. When the original series began in 1966, series writers determined a Vulcan’s blood would be copper-based, thus be green, probably based on copper roofing oxidizing to green. On Earth, arthropods such as snails, spiders, and octopi have copper-based hemocyanin systems which produce blue blood. The writers undoubtedly overlooked this fact in the speculative fiction series or extrapolated that alien hematology was beyond human knowledge at the time. Thus Spock’s blood is green, defying our current understanding. Spock’s blood type is T-negative, considered rare on the planet, similar to AB negative, the rarest blood type among humans.