The players had just ventured down the tunnel and into their respective dressing rooms for half-time when the referee, Guido Winkmann, suddenly ordered them all back out onto the pitch.
The penalty was given for a handball by Marc-Oliver Kempf that the VAR noticed after the halftime whistle had blown.
Referee Guido Winkmann, who'd allowed play to continue, then blew for half-time and was on his way off the pitch when notified by video referee Bibiana Steinhaus, who was watching in Cologne, that he should check a replay. One Freiburg player could even be heard saying: "No, we're not coming back out".
De Blasis, who'd had to wait for Alexander Schwolow to re-take his place in goal, held his nerve to score inside the left corner as the Freiburg goalkeeper dived the other way.
Mainz and Freiburg are 15th and 16th in the table respectively so the decision carried added weight in a relegation six-pointer.
There was a further 10-minute delay before the start of the second half as fans threw hundreds of toilet rolls onto the pitch in a protest against Monday evening matches.
The Freiburg keeper gifted Mainz possession inside his own penalty area, and Colin Quaison squared the ball to provide De Blasis with an open goal.
The game itself may have been otherwise unremarkable, but that didn't stop the fans from creating a spectacle of their own as they continued to display their displeasure with Monday night games.