The wine so called does not receive its name from its colour, but the colour so called receives its name from the wine. The word means clarified wine (vinum claretum) What we call hippocras was called claretum, made of wine and honey clarified.
Blood. To broach one's claret. To give one a bloody nose; so called from the claret colour.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894