Death of a Travelling Salesman

S1 E3: When a travelling salesman dies in a single car accident on a quiet country road, Chief Superintendent Lawson labels it a drink driving case. But when Constable Parks reveals the corpse’s face was blue, Dr Blake is intrigued. Although the victim’s clothes reek of alcohol, Blake concludes that the salesman was not drunk and certainly died before the crash. But how? And why is the face blue?