"Because the Conservative Party was his life's blood, you know," said Drummond. "Oh yes. He'd bleed blue".
"Видите ли, Консервативная партия была нужна ему как воздух, – продолжил Драммонд. – Да-да. Он был с нею одних – голубых – кровей."
Obvious choice for bleed blue, of course; but a British reader would know that blue is the color of the UK Conservative party (same as for US democrats, although possibly a different hue, I didn't check). A concrete political reference changed for a general idiom.