The Blues trailed 16-10 with just a minute remaining but a try from Spriggs and a conversion from Jarrod Evans saw them seal a dramatic victory in driving rain.
After four pre-season wins, the Blues went into the game in confident mood but struggled to implement their expansive style of play in terrible conditions.
"All the training we've done in pre-season on the way we want to play the game went out the window," said Blues performance manager Richard Hodges.
"The hardest thing for us was the weather, they were horrific conditions and we couldn't play the rugby we wanted."
Before Spriggs' last minute try, winger Ellis Wyn Benhan also got himself on the score sheet and Tom McAloon added a conversion and a penalty but Hodges admitted his side were lucky to get the win.
"If we are brutally honest the Dragons probably deserved the win," he said.
"But I've got to give great credit to the boys to grind out the win in those circumstances but if we hadn't won we couldn't have complained.
"The pouring rain made it tough to get any momentum, in hindsight we could have called it off and although the evening forecast was dry, once we kicked-off the heavens opened."
The Blues take on the Scarlets in their next fixture in two weeks' time with Hodges expecting a difficult game away from home.
"It will be a tough game away, it'll be their first game but they've had a good pre-season too," he said.
The other Under 18 match between the Ospreys and the Scarlets was postponed due to bad weather. Heavy rain at Llandarcy saw the pitch become waterlogged and the match has been rearranged for a later date.