Corroded pennies turn green. Gene Roddenberry remembered this from his childhood spent digging them out of fishponds and incorporated into his Spock biology. Thus green blood means it is copper based.
Then he got it wrong. See my horseshoe crab posting above.
Well, Gene was never one for scientific or historical accuracy, or even plausibility. (see the old episodes with the gangster planet and Nazi planet for some nice examples) Apparently, he just wanted to tell a story... ironically, his show spawned one of the most tech-minutiae-obsessed fanbases around. Go figure.![]()
A-freakin'-men.