American surgeon who, with pediatric cardiologist Helen B. Taussig, devised a surgical treatment for infants born with the condition known as the tetralogy of Fallot (Fallot, tetralogy of), or “blue baby” syndrome.

After graduating from the University of Georgia in 1918 Blalock entered the Johns Hopkins University School of Medicine, from which he received his M.D. degree in 1922. From 1925 to 1941 he was a resident in surgery in the school of medicine of Vanderbilt University. During that time he conducted research on traumatic and hemorrhagic shock; his conclusion that the effects of shock were due to loss of blood volume led to the volume-replacement treatment that was credited with saving countless lives during World War II.

Blalock returned to Johns Hopkins in 1941 as professor and head of the department of surgery in the school of medicine and as surgeon-in-chief of the Johns Hopkins Hospital. In collaboration with Taussig (Taussig, Helen Brooke), Blalock devised a procedure known as subclavian-pulmonary artery anastomosis, by which the congenital heart defect that produced the “blue baby” syndrome could be corrected and the patient enabled to lead a nearly normal life. The first such operation was performed by Blalock in 1944.