Dr. Blain was born to missionary parents in Kuching, China. At the age of 11, he attended school in Shanghai, and at 13 years old, he came to the US. He received his M.A. in 1921 from Washington and Lee University and an M.D. in 1929 from Vanderbilt School of Medicine. He interned at Peter Bent Brigham and City Hospitals in Boston (1929–31). He took a year of residency at Boston City Hospital, followed by service at the Austin Riggs Foundation in Stockbridge and Silver Hill, a private mental hospital in Connecticut. He was in private practice in Greenwich, Connecticut, and New York City (1933–42).
During WWII, Dr. Blain was commissioned into the U.S. Public Health Service and appointed Medical Director of the War Shipping Administration to provide medical services for members of the Merchant Marine. At the end of the war, he was named Chief of the Department of Psychiatry and Neurology in the Veterans Administration in Washington, DC.
In 1948, Dr. Blain became the first Medical Director of the American Psychiatric Association, where he served for ten years. He then spent a year as Director of Research and Training at the Western Interstate Council for Higher Education in Colorado, followed by serving as Director of Mental Health for the State of California (1959–63). He spent the following three years at Pennsylvania Hospitals in Philadelphia as the Director of the Community Psychiatric Program (1966–70), and he was Director of the State Mental Hospital in Philadelphia.
Dr. Blain served as Chairman of the Department of Psychiatry at Georgetown University Medical School (1945–48), and (1958–1971), he was on the faculty of the University of Pennsylvania. In 1975, he received the Salmon Medal from the New York Academy of Medicine. He was a member of numerous psychiatric organizations, including the American College of Psychoanalysis, the American Psychoanalytic Association, the American Psychopathological Association, and many others. He made contributions to the psychiatric literature.
Dr. Blain served as President of the American Psychiatric Association (1964–65).