He started the Mets’ 4-1 Grapefruit League loss to the Marlins on Wednesday and tossed a clean 1-2-3 inning with a strikeout and two ground outs.

It marked his first outing since suffering a cerebrospinal fluid leak late last season and undergoing surgery — in which doctors shaved bone spurs around a capsule in his spine that was punctured, drained spinal fluid and stitched the capsule — in October.

Blackburn needed just seven pitches — all strikes.

The plan was for one inning, so his quick frame prompted him to throw 15 more pitches in the bullpen to get to the team’s desired workload for him.

“Huge day,” manager Carlos Mendoza said. “It’s a big day for him, it’s a big day for us, because it wasn’t easy what he went through. It was pretty scary when he got the news. And the fact that it’s not even March — only one inning today, but he’s in live competition, facing another team. Huge step for him, proud of him. It means a lot that he’s pitching in games this early.”

Frankie Montas (lat strain) and Sean Manaea’s (oblique strain) injuries have put Blackburn firmly in the mix to earn a rotation spot.

The Mets acquired the 31-year-old Blackburn from the A’s just before the trade deadline last year in exchange for minor league pitching prospect Kade Morris.

In five starts with the Mets, he recorded a 5.18 ERA before being shut down.

“I had the normal jitters and a little anxiety before the game, just wanting to start, some excitement,” Blackburn said. “I feel like today was a good step in the right direction.

“I feel good. I feel good out there, feel like I can move the ball, feel like I can spin stuff like I normally have. Everything feels good out there.”

Kodai Senga, Clay Holmes and David Peterson appear to be locks for the rotation. That leaves Blackburn, Tylor Megill and Griffin Canning as the next most likely candidates competing for the final two spots on the staff. Neither Megill nor Canning have seen any game action this spring.

Wednesday marked a strong start for Blackburn toward claiming one of those spots.

“I’m extremely excited about this year — kind of put everything from last year behind me,” Blackburn said. “Feel like I’m in a good spot right now.”