When he arrived to Citi Field on Wednesday, Blanco said he still felt "different" and was scheduled to undergo more concussion tests.
"I think in general if you feel like something's not right, you don't feel normal, that's a good indicator," Belt said. "That's where he was."
The move is retroactive to Tuesday, and although the Giants are not sure of the severity of the injury, they are hopeful Blanco will be cleared when eligible to come off the disabled list.
"I think these are tough calls and they can be hard to read," manager Bruce Bochy said. "We're optimistic right now that he'll be ready to go on Tuesday, but I don't think you know until you get a couple more days in this and see how he's feeling."
Outfielder Jarrett Parker was promoted from Triple-A Sacramento to take Blanco's place. He was hitting .268/.382/.514 in the Minors with 12 homers and 36 RBIs.
"He gives us a true extra outfielder, good athlete, can play all three positions," Bochy said.
Parker was promoted last season when the team was in New York, but spent just one day in the Majors before he was sent back down.
"I guaranteed him two days this time," Bochy said with a laugh.
• Hunter Pence took the day off from baseball activity Wednesday, but could be headed to the disabled list Thursday, which could be retroactive to June 2. Bochy said the Giants would likely add another hitter in that case.
• Bochy said Matt Cain topped out at 94 mph on the radar gun during his rehab start Tuesday night. Cain will make his next rehab start for Sacramento on Monday.
Jamal Collier is a reporter for MLB.com. Follow him on Twitter at @jamalcollier. This story was not subject to the approval of Major League Baseball or its clubs.