German champion Bayern Munich will have Donovan from Jan. 1 until the start of the MLS season in mid-March.
Blanco will join Santos for the Mexican league playoffs, which start this weekend with the first leg of the quarterfinals.
Donovan has spent a week and a half training at Bayern and scored two goals for the "A" team (minus its players on international duty) in a scrimmage against the "B" team on Wednesday.
"Landon has a verbal agreement with MLS to play in Europe for two-and-a-half months, like Beckham's," Bayern coach Juergen Klinsmann said in reference to the agreement Donovan's Galaxy teammate has to play for AC Milan. "He's made such a good impression here that we've agreed to talk to MLS. The goal will be to see if we can get him long term, at least until the end of the season."
The permanent transfer of Donovan would probably require Bayern to exceed the record American transfer fee of $10 million Spanish club Villarreal paid for 18-year-old Jozy Altidore over the summer.
Bayern would probably need to first unload unhappy Lukas Podolski. Donovan gives Klinsmann a fourth striker as backup for Luca Toni, Miroslav Klose and Podolski.