Alison Walshe shot a 4-under 68. Australian Stephanie Na and Canadian Stephanie Sherlock are three shots back entering Tuesdays final round of stroke play.
The field of 156 will be cut to 64 for five rounds of match play beginning Wednesday.
Blumenherst, the three-time reigning NCAA Player of the Year from Duke, was the runner-up at the U.S. Womens Amateur last year, falling 1-up to Maria Jose Uribe of Colombia in the championship match. Uribe played in the Womens British Open last week and is not defending her amateur title.
Starting her round on No. 10, Blumenherst had her only bogey on the par-4 No. 15. She then played her last nine holes at 5 under, sinking back-to-back birdie putts to conclude her round.
Walshe made the turn at 3 under before an adventurous back nine, which included four birdies and three bogeys. She had three consecutive birdies, starting on No. 15, before finishing with a bogey 5 on No. 18.
After completing her senior season at Arizona this spring, Walshe went 4-0 to lead the United States to victory at the Curtis Cup at the Old Course at St. Andrews in Scotland. She then tied for 31st at the U.S. Womens Open in June.