Blues South under 16s took the honours on derby night as the Blues North suffered a fourth defeat of the season in the WRU Age Grade Championship.
Blues North, who defeated the Ospreys at Llandarcy last time out, failed to bag their second win of the season despite a Jack Barratt try and eight points from the educated boot of centre Callum Phillips.
Still, the night belonged to South wing Osian Davies who swept in for a try hat trick. Gareth Thompson added 10 points with the boot to make it four wins in six games.
The two sides were well matched for long periods of the game, but Davies took his opportunities well and that was the difference.
The Scarlets under 16s edged past the Ospreys to stay on top of the pile.
However, the hosts were made to work for their 17-13 success whilst in the other game, Blues South defeated Blues North 25-13 at Taffs Well.
Although they trailed 13-12 at half time, the Scarlets won the day with a single try in the second half from wing Steffan Evans.
Before the interval, Jake Randall and Gethin Davies crossed for the Scarlets with leading scorer Jordan Williams adding a conversion for good measure.
However, it was the Ospreys who led at half way courtesy of tries from Sam M Davies and Rhys Williams and a Sam Davies penalty.
Scarlets team manager Wyn Thomas said; "It was an excellent game and could have gone either way.
"We trailed narrowly at half time but Steffan Evans' try put us back in front two minutes into the half. After that, the Ospreys pressed hard, but our defence was good and for that alone we deserved to hold on."
Cardiff Blues travel to Myreside on Friday to take on Edinburgh Rugby in a crucial Guinness PRO12 clash, and with Wales facing Scotland in the RBS 6 Nations at BT Murrayfield on Saturday, it promises to be a weekend of non-stop rugby action at the Scottish capital. Here are some useful advice to Cardiff Blues travelling supporters.
To celebrate the opening of the newly refurbished Guinness PRO12 Pack Bar, the world famous Irish brewer will be offering supporters free pints of the black stuff ahead of Saturday's clash with Ulster.
'TACKLE' - the first-ever joint community rugby initiative between the four professional teams in Wales has been launched (today Weds 19th October) bringing an inspiring new rugby education programme to young people in Communities First areas. Partners in the project were joined by Wales' star rugby internationals Sam Warburton (Cardiff Blues), Alun Wyn Jones (Ospreys), Gareth Davies (Scarlets) and Hallam Amos (Newport Gwent Dragons), for the launch at Aberdare Community School.
The Welsh Rugby Union, together with Pro Rugby Wales, has secured significantly improved insurance cover for all registered players, coaches and referees at each of its member clubs and regions at reduced costs -working with Aon, one of the world's largest insurance brokers and risk advisors.