Witherspoon, 24, spent last season at Double-A Arkansas, where he hit .214 with 18 doubles, 11 homers and 30 stolen bases.
The 42-year-old Blanco appeared in 35 games for the Mariners this year. He was brought in to help fill the catching role after injuries and hit .125 with Seattle. A 16-year big league veteran, Blanco started the season in Toronto.
Seattle announced the moves Tuesday.