And having worked the ball from deep inside their own 22, they were rewarded with a try from replacement Steffan Phillips. Dale Ford converted to make sure of a 15-12 victory which consolidates the Scarlets' lead at the top of the ladder.
Earlier in the game, the Blues had taken a 12-3 lead courtesy of tries from Elliott Jones and Owen Williams plus a conversion from Diggy Bird.
Sam Parry's try a minute into the second half kept alive an outstanding contest and a penalty from Steven Shingler and Phillips' late converted try won the day.
Scarlets Under 18 manager Cerith Williams said; "It went right down to the last play, but fair play to the boys, they never let their heads drop. In the end they were rewarded for their efforts. It was a crucial victory for us."