Flanker Alex Murphy added a third try in the second half and Flannagan converted for 24-0.
Meanwhile at under-18 level, the Blues found old rivals the Dragons too hot to handle at Sardis Road - the Dragons running out 38-16 winners.
Jack Bird opened the scoring for the Blues with a first-half try, whilst Christian Ballor converted and added three penalties.
Blues South under 16s got off to a winning start, against the Ospreys at Pentyrch thanks to a late penalty from outside half and captain Phil Williams. However, it was not such a good start for Blues North who went down 27-15 to Llanelli Scarlets at Aberdare.
Mike Powell and Ashley Macey crossed for the Blues' tries with Luke Davies adding a penalty and Owen Davies a conversion.
for the contact details for Cardiff Blues regional coaches or visit www.wru.co.uk for more information on age grade rugby in Wales.