You have used all your guesses.
SACRE was sought.
Canvas is traditionally white. You are not traditional. - History ref: Traditions are passed from parents to their children. Sacre's mother is pure white in colour, he is not. A traditional unicorn also has a lion's tail and a beard, he has a 'normal' tail, no beard.
Or maybe you are? Blood runs thicker than water. - History ref: He is still a pure unicorn, with pure unicorn parents.
Magic ref: He also has magic that involves blood.
Appearance ref: He has a blood marking on his flank.
Do you ever wonder what would become of you? - Fluff.
Your name is Robert. - Name ref: When googled, Sacre results in multiple Robert Sacre entries.