Blumenherst birdied the last five holes of the Industry Hills Golf Club course for a 7-under 66.
Sandra Gal was alone in second at 67 and Michelle Wie fired a 68 to take third place. Only 14 players broke par, with Mika Miyazato and Catriona Matthew among those who shot 70 and Christina Kim included in the group at 71.
Blumenherst is seeking her first tour victory. She missed the cut in seven of her last 12 appearances in 2010 and has finished no better than 43rd in three tournaments this year.