Blood is a combination of certain types of specialist cells suspended in a fluid which is found in most corporeal lifeforms. Blood circulates the body delivering nutrients to cells and taking away waste products.
Depending on the base material of the blood it can be different colours for different species. Many species, including Humans had red, iron based, blood. Vulcans and Romulans had green copper based blood, whilst Andorians and Bolians had blue cobalt based blood. This often resulted in cultural significances of the colours, for instance: on Human starships a red alert signified danger, while on Romulan ships red was a good omen-with green lighting indicating danger. Such was the case when James T. Kirk and Leonard McCoy were lost in the decks of a warbird, after overpowering their Reman medic, Facilitator. The two figured out crimson was "safe" and emerald implied a warning. (ENT novel: The Good That Men Do; TOS novel: Captain's Blood)
Blood cells were some the first to be assimilated when an individual was injected with Borg nanoprobes, the nanoprobes found blood cells and latched onto them, taking control. This process caused the grey, veiny appearance-which signified someone has just begun the process of being assimilated. (VOY episode: "Scorpion"; TNG movie: Star Trek: First Contact; et al.)
The inhabitants of Meldrar I held a tradition in which, following the death of a member of their race, a pouch of the individual's blood was returned to Meldrar to be poured into the Meldrarae Sea. Members of the Andorian Guard had a similar tradition. These Andorians would take a vial of blood from a comrade who died away from Andoria back to the homeworld and leave it at the Wall of Heroes. (DS9 - Avatar novel: Book One; ENT episode: "United")