Dortmund had been hoping to keep the pressure on the top two and second-placed RB Leipzig ultimately lost 3-0 at home to Hamburg, but Tuchel's men failed to capitalise.
Antonio Colak scored the decisive goal in the second half for Darmstadt and it was nothing less than the hosts deserved, as they carved Dortmund open on numerous occasions.
The win was Torsten Frings' first in charge of the club since becoming coach at the end of December and Darmstadt's first since October.
"It was a deserved victory for them," Tuchel acknowledged to reporters. "If we play so far below our limit, it is normal to lose the game.
"It makes sense that the team which plays to its full potential wins. We were deserved losers today.
"I never had the feeling today that we ever really got into the game."
Frings was unsurprisingly elated at the full-time whistle and he lavished praise on his players, particularly Hamit Altintop, whose signing at the end of January was the subject of criticism.
"All the players were outstanding today," Frings said. "And some people should apologies to Hamit Altintop.
"Even when it was 1-1 we wanted to play and be brave. I'm incredibly proud of the troops. It's great that the team has finally rewarded."