The 1930 census shows a Doris May Blanton with father Wendall and born in Missouri, living in St. Louis, Mo. Her age was 8 years as of Apr 1, 1930, consistent with either birth date. They were living with her grandmother.

A search turned up a birth record for her at Archives.com, but you have to be a member to get it.

She can order a MO birth certificate at:

http://health.mo.gov/data/vitalrecords/

Also, at the Recorder of Deeds Office in St. Louis, if born there.