Along with his night school art lessons and his early days as a commercial art studio apprentice, the young Haddon Sundblom had some other extremely important influences that informed his painting technique.
Among others, Howard Pyle, John Singer Sargent, Robert Henri, Anders Zorn and Joaquin Sorolla were all practitioners of a kind of painting adapted from the Impressionists called "alla prima" or "first stroke," The technique involved "laying down the fewest strokes in the quickest time to sufficiently describe moving targets," as Roger T. Reed explains in a fascinating, informative article on Sundblom at the Illustration House website.
Sundblom acknowedged Zorn as his principle influence but in his June '56 article in American Artist, author Frederic Whitaker writes, "There never could have been such a Sundblom had there never been a Howard Pyle, for the Pyle concept is easily seen in the Chicago artist's work."
Whitaker further credits Sorolla for "[unlocking] for Sunny the secret of the sun-lit glow that pervades all his work."
Other painter/illustrators who Sundblom acknowledged as being an influence on his style include J.C. Leyendecker, Pruett Carter and Walter Biggs.
* I encourage you to read the Sundblom "all prima" article by Roger T. Reed at The Illustration House website - its really explains the topic succinctly and includes a terrific gallery of early Sundblom paintings.
* Many thanks to Tom Watson, who generously provided all of today's scans!