from the exhibition guide:
The chough appears black on the blog, and will appear white in the cathedral. Through this device, the chough as a symbol of Cornwall, also comes to symbolise good/evil and life/death.
Black is the colour of mourning: white is worn at weddings and christenings.
A black/white dichotomy appears in literature and in the Bible: most obviously in the story of Noah, when Noah sends out both a raven and dove.
It also occurs in Tristan and Isolde, a myth set in Cornwall, where white and black boat-sails are used as symbols of life and death.