The River Cats announced the move Monday on Twitter, then A's assistant general manager David Forst said Blanton would start with Sacramento for now.
Blanton was granted his unconditional release by the Los Angeles Angels last Wednesday, and they still owed him $8.5 million.
The 33-year-old Blanton struggled in spring training with a 7.08 ERA in five appearances over 20 1-3 innings. He was 2-14 last year and sent to the bullpen in midseason.
Blanton was due $7.5 million this year, and had an $8 million club option for 2015 with a $1 million buyout.
Oakland opens the season Monday night against the Indians.