The M's found themselves behind 2-1 in the seventh, but Brandon Ryan coaxed a two-out walk from Orioles starter Wei-Yin Chen and Blanco then pulled an 0-2 pitch down the left field line and into the seats for the decisive blow.
Joe Saunders (10-10) turned in six solid innings to earn the victory, yielding two runs on eight hits. He walked two and struck out two.
Yoervis Medina tossed two shutout relief innings and Danny Farquhar corralled his second career save as the Mariners took two-of-three games against the Orioles.
Chen (6-4) absorbed the loss, surrendering three runs on five hits over seven full innings.
Danny Valencia cracked a two-run homer for third-place Baltimore, which fell 6 1/2 games behind American League East-leading Boston.