Chase Blackburn spent most of the spring unemployed in Ohio, working out in a gym and doing defensive and special teams drills by himself at a local park. He kept studying his Giants playbook, waiting for a call from Big Blue, or anybody else. Once the Giants finally brought Blackburn back last week, after seeing his performance against Green Bay, one has to wonder what took them so long.

Blackburn had five tackles and an interception in Sunday’s 38-35 loss to the Packers, and — going into this weekend’s huge game at NFC East-leading Dallas — he has a chance to show the Giants they should’ve brought him back sooner.

“I’m going to try and prove that it was a mistake for me not playing earlier this year,’’ said Blackburn, who hadn’t put on pads or a helmet in 13 weeks, but got 50 or 60 snaps against the Packers.

“I was sitting on the couch before I got picked up my rookie year; I waited a long time. I was just trying to get the opportunity and take advantage; that’s what I’ve tried to do again, the same situation. I didn’t have a spot on any team; no one was wanting to take a chance on me. Now I’m just trying to prove it was a mistake.’’

Blackburn had been the Giants’ special-teams captain last year, but they didn’t re-sign him after a tryout earlier this season. He insisted he was 100-percent healthy, just working and waiting for his chance; it came with Michael Boley just recently working his way back from hamstring woes and Mark Herzlich sidelined with a bad ankle.

“He had his bags packed quite a while before he actually got the phone call. But he comes in, it’s like he never left, her jumps right in there and no matter what you ask him to do, he’s ready to go,’’ said coach Tom Coughlin, who had Blackburn calling defensive plays in the second half.

“He’s a student of the game. It looked like he never missed a beat,’’ said Antrel Rolle.