Hernandez (8-4) scattered five hits and walked just one batter while striking out eight en route to his third win in four starts.
"I just tried to do my job and throw good pitches. At the beginning of the game I was a little wild. After that, I was way better," said Hernandez.
Michael Morse went 3-for-4 with a pair of doubles and Nick Franklin also stroked a two-bagger for the Mariners, winners in four of their last five games.
A.J. Griffin (5-6) gave up eight hits, including Blanco's grand slam, over six innings to absorb the loss.
"To go 5 1/3 innings scoreless and give up a grand slam like that is really frustrating. Hopefully we don't have to talk about something like this again," Griffin said.
Jed Lowrie opened the bottom of the fifth with a double and moved to third on Seth Smith's fly out to center. Chris Young then lifted a fly ball to right field, and Lowrie broke for home. Endy Chavez, though, delivered a strike to cut down Lowrie at the plate to keep the game scoreless.
Blanco delivered his second career grand slam, and his first since 2000, in the sixth. Kendrys Morales singled then moved to third on Morse's double to right field. After Griffin intentionally walked Michael Saunders, Blanco smashed a first-pitch fastball over the wall in left field.
Oakland threatened in the seventh after the Yoenis Cespedes and Brandon Moss opened the inning with back-to-back singles, but Hernandez fanned Josh Donaldson before getting Lowrie to ground into an inning-ending double play.
The A's had one last chance in the ninth. Reliever Charlie Furbush gave up a lead off single to Coco Crisp, spelling the end of his night. Yoervis Medina came on to replace Furbush and promptly walked Derek Norris to put runners on first and second with no outs. Medina, though, retired the next three hitters in order to nail down the win.
Hernandez has lasted at least seven innings in each of his last four starts and nine of the 15 times he has toed the rubber for Seattle this season ... The Mariners will send Hisashi Iwakuma to the mound on Sunday as they look to complete their first series-sweep of the season ... Oakland dropped back-to- back home games for the first time since losing two in a row to the Texas Rangers on May 14 and 15 ... Bartolo Colon will start for the A's on Sunday.
06/15 23:51:07 ET