Hernandez (8-4) dominated the Athletics by scattering five hits and a walk and striking out eight in posting his third win over four starts.
The Mariners' ace placed 68-of-108 pitches for strikes and improved to 15-6 all-time against Oakland as Seattle took a second straight game from the American League West leaders.
The 41-year-old Blanco, playing in his first game for the Mariners since being signed Friday, connected for a homer with Kendrys Morales, Michael Morse and Michael Saunders aboard in the sixth inning for the game's only runs.
A's starter A.J Griffin (5-6) was responsible for all four runs. He gave up eight hits in six frames.