Mainz initially had the penalty appeal turned down after right-back Daniel Brosinski's cross deflected off Freiburg centre-back Marc-Oliver Kempf's hand and was saved by keeper Alexander Schwolow.
Referee Guido Winkmann initially allowed the game to play on, denying Mainz a penalty, before blowing for halftime shortly thereafter.
He ordered the players back onto the pitch much to the bemusement of Freiburg coach Christian Streich, who was seen shaking his head in disbelief.
"It leaves a bitter taste, but we have to accept it with a heavy heart", Freiburg sporting director Jochen Saier told Eurosport at half-time.
The players then returned to their dressing rooms, however the fans weren't quite done as Mainz supporters suddenly started hurling toilet paper onto the pitch, meaning the second-half was delayed even further as the stewards were left with an almighty clean up.
Before the game began, both sets of fans waved signs reading "against Monday games" and a cocophony of whistles and vuvuzelas provided a constant soundtrack throughout the 90 minutes.
Mainz and Freiburg are 15th and 16th in the table respectively so the decision carried added weight in a relegation six-pointer.
De Blasis followed the penalty, officially recorded in the seventh minute of first-half stoppage time, with a second goal in the 78th minute to seal the win. Wolfsburg is also under threat with four rounds remaining, while survival looks increasingly unlikely for Hamburger SV and Cologne, eight and nine points adrift of safety, respectively.