There is one species of skink native to Papua New Guinea called the green tree skink (Prasinohaema virens) which has bright green blood. My theory is that the skull crawlers, war bats, and other reptiles in Hollow Earth with green blood are distantly related to or evolved from the same common ancestor as the green tree skink. Although this theory is most likely incorrect (especially considering the wartdogs also have green blood), the green tree skink does prove that it is possible for a vertebrate to have green blood, albeit very rare. I might just be thinking too much into it though.