Moriya Jutanugarn also had a 66 at the Bay Course of the Seaview Hotel and Golf Club to add to what has been a solid rookie season on the LPGA Tour. Jutanugarn opened the year by finishing fourth at the Australian Open.
Blumenherst is in her fourth season on the circuit and while she has had six Top 10 finishes in that time, she has yet to post a victory. She is married to Nate Freiman, who is in his rookie season as a member of the Oakland Athletics.
Defending champion Stacy Lewis was alone in third at 67 while Michelle Wie shot a 68 to finish by herself in fourth place. Wie has missed the cut in four of her nine tournaments this year and her best showing has been a tie for 28th.
Feng Shanshan, Jennifer Song and Beatriz Recari were among the seven players who opened with a 69.
World No. 1 Inbee Park struggled to a 74.