However, Darmstadt could have rescued a draw when they were awarded a penalty deep into second-half injury time, only for Sandro Wagner to blaze his effort high over the top of goal.
Despite dominating much of the possession, it was the away side's first effort in the game, but their second effort on 24 minutes yielded another goal as Yunus Malli curled home from the edge of the box.
Their two-goal advantage lasted less than two minutes, however, as Marcel Heller collected the ball on the edge of the box and turned beyond his man before firing into the top corner to reduce the deficit.
However, just as Dirk Schuster's team had wrestled their way back into the game, they found themselves behind again when they failed to clear the ball, allowing de Blasis to fire home.
Christian Mathenia then had to be at his best to paw away a rasping shot from Yoshinori Muto, and Wagner's free kick deflected narrowly wide, before Wagner blew his chance from the spot in the 96th minute.
The result sees Mainz climb to fifth place in the Bundesliga, with 12 points from their opening eight games.