The Blues bucked up in the second half, but some outstanding defence from the Dragons, most notably Marcus Johnston whose last ditch tackle on scrum half Jack Davies prevented a certain try.
Gafyn Cooper, director of development within the region, said, "It was a poor game, but to the credit of both sides, they kept going in difficult conditions.
"There were a lot of mistakes, by both sides, but the Dragons probably just about deserved it," he said.
In the other game The Ospreys threw wide open the title race with a 15-3 defeat of the Scarlets at Dunvant.
Having lost out to their closest neighbours on the opening night of the campaign, the Ospreys reaped revenge and closed the gap on the leaders.
Tonight the Blues U20's will hope to complete the double over the Dragons after beating them 24-7 on the opening night of the competition. The game has been switched to Abercynon RFC due to a waterlogged pitch at Glamorgan Wanderers RFC. K.O. 7:15PM.