However, a penalty try in the closing minutes gifted the Ospreys a significant victory.
Adam Powell, Rhys Shellard and Tom Riley bagged tries for the Blues with Gareth Williams-Davies adding two conversion and three penalties.
However, it was the Ospreys' night and tries from James Hook, Tim Stephenson, Ben Lewis and that late penalty try provided the victory. Hook added three conversions and a couple of penalties to make sure.
Llanelli Scarlets proved too strong for the Newport Gwent Dragons, winning 38-7 at Stradey Park.
Rhydian Thomas opened the Scarlets' account and further tries from Craig Evans (2), Stephen Davies, Ian Jones and David Halpin made sure of the points.
Aled Thomas bagged four conversions whilst for the Dragons Greg Callow converted his own consolation try.
The Dragons suffered a surprise 23-15 defeat at the hands of Llanelli Scarlets in the second round of the under 18s competition at Abertillery Park.
Penalties from Rhys Priestland on two an 15 minutes gave the Scarlets a deserved lead and when centre Rhys Williams hacked and picked up to score seven minutes later, the Dragons were staring down the barrel of a surprise defeat.
Rhys Jones cut the deficit to 10 points just before half time, but when Huw Gustaffson broke from a ruck three minutes into the second half, the visitors were back in complete command.
Priestland's conversion made it 20-3 and though Ben Flowers burst through to score a try that Jones converted, Martin Thomas replied with a penalty to redress the balance.
Robert Lewis dashed over in the final minute for a try that Jones was unable to convert, but it was too little too late. Tries from Rhydefelin's Lee Scarf and Jonathan Gough of Ynysybwl were enough to earn the Blues under 18s a 14-8 victory over Neath Swansea Ospreys.
Alex Wilkins with a try and a penalty from Nicky Roberts put the Ospreys in a decent position, but Scarf's first half try and a second, after the break, from Gough made sure of the points.
A penalty from Phil Williams was not enough to earn Cardiff Blues South under 16s a second successive victory of the season at Stradey Park, they lost 14-3 to the Scarlets for whom Matthew Lemon (2) and Geraint Peachment scored tries.
A try from Jason Tovey of Cwmcarn School and Risca Rugby Club, plus a conversion and a two penalties from Abertillery's David Langdon should have been enough to earn the Dragons under 16s victory over the Ospreys at Maesteg.
Unfortunately, the Ospreys, who lost their opening game against Blues South last month, snatched a 14-13 victory from the jaws of defeat courtesy of an injury time try from hooker Dean Ronan.
Llanelli Scarlets 14 Cardiff Blues South 3
Neath Swansea Ospreys 14 Newport Gwent Dragons 13
Newport Gwent Dragons 15 Llanelli Scarlets 23
Neath Swansea Ospreys 8 Cardiff Blues 14
Cardiff Blues 28 Neath Swansea Ospreys 32
Llanelli Scarlets 38 Newport Gwent Dragons 7