Darnell finished with two hits and Jason Bartlett added two hits and an RBI for San Diego, which avoided a three-game sweep at Great American Ball Park.
Wade LeBlanc (1-2) limited the Reds to three runs on two hits over six frames, recording his first victory of the season.
Reds starter Dontrelle Willis (0-3), meanwhile, was rocked for four runs over 2 2/3 innings before exiting with forearm soreness in his left pitching arm.
The team said the veteran hurler will be evaluated Monday.
Cincinnati saw its four-game winning streak snapped despite a three-run homer from Jay Bruce.