Cole Perfetti gave the Jets a 1-0 lead on a rebound in the slot at 14:57 of the first period.
Mercer responded 19 seconds later to tie it 1-1. After controlling a loose puck in the slot, Mercer attempted a backdoor pass to Tomas Tatar at the left post, where Winnipeg defenseman Nate Schmidt inadvertently pushed it into his own net.
"I passed it over there, two of them were there, and I was just happy to see the puck go in, celebrate it," Mercer said. "It didn't matter who put it in as long as it went in. That that's all that matters."
Pionk put Winnipeg back in front 2-1 with a slap shot from the point at 17:09 of the first.
"We were playing a pretty good game, keeping them to the outside and letting me see pucks," Hellebuyck said. "In playoffs, that's where we get hard-nosed and kill time. Tonight it was a couple turnovers, and they capitalized."
Blackwood kept the Devils within one at 14:48 of the second period, when he made his best save of the game with his left pad against Scheifele on a rush down the left wing.
"I like where my game is at and, obviously, I'll continue to try and build it every game, every practice," Blackwood said. "But it feels good to get a win."
Said Ruff: "We're going to need both goalies (Vitek Vanecek) because the schedule gets real demanding. [Blackwood] fought through that first period and, I thought, did a heck of a job."
NOTES: Jets forward Nikolaj Ehlers left with an undisclosed injury after getting knocked to the ice at 11:58 of the third period. There was no update postgame. ... Devils forward Jack Hughes had his 10-game point streak end. ... New Jersey is 14-for-14 on the penalty kill during its six-game home winning streak.