Given the severity of his wounds, would he have received a blood transfusion on the battlefield? How likely is it that blood transfusions would have been available to the 369th regiment? I couldn’t find any details on the treatment of his wounds.
… Johnson repelled the Germans, killing four while wounding others, rescuing Needham Roberts from capture and saving the lives of his fellow soldiers. Johnson suffered 21 wounds during the ordeal.[[3]] This act of valor earned him the nickname of “Black Death”, as a sign of respect for his prowess in combat.
That is what the wikipedia article suggests. Blood transfusion was still experimental in the early 20th century. WWI hastened the development of the technique, but I doubt that anything could be done on the battlefield.