Blanch was born at Viney Hill Farm, Blakeney, Gloucestershire in the Forest of Dean, the youngest of three sons of a farmer, William Edwin Blanch, and his wife, Elizabeth, "née" Yarworth. William Blanch was killed in a shooting accident in 1923 and his widow and youngest child moved to London, where the two older sons were already living. Blanch attended Alleyn's School, Dulwich, winning a scholarship after the first year. His mother could not afford to pay for him to go to a university, and after leaving school at the age of 18 he started work. He would have preferred to become a journalist, but found that "journalism in particular was hard to get into without contacts." He gained employment in the office of the Law Fire Insurance Society Ltd in Chancery Lane at a salary of £90 a year. He said later, "The job taught me a great deal, not just about administration – how to write letters and so on – but how to deal with people from all walks of life."