However, the Dragons recovered to lead 15-13 at half time courtesy of tries from centre Hallom Amos and wing Ashton Hewitt and a conversion and penalty from outside half Lloyd Rogers.
Llewellyn edged the visitors back in front with a penalty in the early stages of the second period, but the Dragons, who had prop Luke Garrett and lock Derryn Jones yellow carded, won the day when full-back and captain Mitchell Starr raced over late on.
Blues player development co-ordinator Gafyn Cooper said; "It was a decent game played out between two well matched sides.
"Obviously we made a good start and then led again in the second peiod. However, they kept on at us and I certainly wouldn't wish to begrudge them the win."
Dragons assistant team manager Colin Brennan said; "We had to hang in there when we lost the two forwards, but to the team's credit, they did exactly that and were deserving of the win."
Last season's under 16 finalists, the Ospreys and Scarlets, met at Parc y Scarlets with the visitors coming out on top 28-15 courtesy of two tries from wing Dafydd Howells and one apiece from centre Joe Thomas and wing Alex Bidwell. Outside half Josh White kicked four conversions.
In a one-sided first half, the visitors used the wind to establish themselves against a plucky Scarlets side who were far from overawed by their hosts.
After the break and with the elements in their favour, the Scarlets got back to 21-15 thanks to tries from wing Matthew Davies and no.8 Lee Taylor and a penalty and conversion from Jack Maynard.
However, the Ospreys finished with a bang and made sure of the points when Howells nipped in for his second try that White converted.
The Scarlets under 18s were too strong for the Ospreys at Llandarcy, winning 36-16 courtesy of 21 points from Jordan Willaims. The outstanding outside half, who starred for the region in last last season's under 16 competition, scored two tries before half time with half back partner Rhodri Williams crossing for the first of his pair.
The scrum half added his second mid-way through the second half and Jake Randall added a fifth late on. Jordan Williams kicked four conversions and a penalty while for the Ospreys, who trailed 22-15 at half-time, there was a try from Ashley Evans and 11 points from the boot of outside half Sam Davies.