San Diego will get minor-league outfielder Jake Goebbert and a player to be named in exchange for Blanks, who has 28 homers in 715 big-league at-bats but has had difficulty staying healthy throughout his career. Blanks has yet to appear in more than 88 games in a season with San Diego.
The Padres optioned Blanks to Triple-A El Paso earlier this week to create room on the roster for Carlos Quentin, who was returning from the disabled list.
Reports said the A's plan to use Blanks as part of a first base platoon with the left-handed hitting Brandon Moss.
Goebbert, 26, broke into the big leagues with Houston as a 13th-round pick out of Northwestern in 2009. He was hitting .257 with six home runs for Oakland's Triple-A Sacramento affiliate this season. He has been assigned to San Diego's El Paso club.
In conjunction with the Blanks move, the Athletics designated first baseman Daric Barton for assignment Thursday. Barton, a career .247 hitter in parts of eight seasons with Oakland, was hitting .158 (9-for-57) in 30 games this season.