Celtic League leaders Neath-Swansea Ospreys edged a tense match at a wet and windy Arms Park thanks to a try in each half.
Blues (9) 9
Ospreys (10) 15
Tries: Pugh, Spice
Flanker Richard Pugh pounced on a loose ball over the Blues tryline to get the first touchdown after 26 minutes.
Blues fly-half Nick MacLeod kept the hosts just one point behind with three first-half penalties.
But scrum-half Jason Spice secured the victory five minutes from time when he darted over from the base of a scrum.
The Ospreys' try came straight from the kick-off after MacLeod's first penalty had put the Blues three point ahead.
MacLeod was engulfed by two Ospreys tacklers on his own tryline as he struggled to gather a loose pass.
The ball was knocked out of his hands in contact and Pugh was perfectly place to touchdown for an opportunist try. Henson converted.
Two MacLeod penalties put the Blues back ahead as the half-time approached, but the Ospreys - who had Matthew Jones sin-binned after 35 minutes - went into the interval with a 10-9 lead thanks to a Henson penalty.
Both defences held firm in the second half but the Blues' resistance was finally broken by Spice as the Ospreys finished strongly.
The scrum-half crossed over after breaking on the blind side from the base of a scrum five meters out.
Pugh thought he had scored a second try when he raced clear down the right wing in stoppage time, but the referee had already blown for a forward pass.
Blues: R Williams; J Vaughton, T Shanklin, T Davies, C Morgan; N
Macleod, R Powell; G Jenkins, R Thomas, B Evans, D Jones, R Sidoli,
K Schubert, M Williams (capt), J Malpas.
Replacements: J Bryant, D Dewdney, J Yapp, G Williams, C Quinnell,
N Budgett, N Thomas.
Ospreys: A Durston; E Seveali'i, S Parker, G Henson, S Williams; M Jones, J Spice; P James, B Williams (capt), A Jones, B Cockbain, J Thomas, A Lloyd, R Pugh, R Jones.
Replacements: D Jones, R Hibbard, A Millward, A Newman, J Bater, R Rees, D Bishop.
Referee: Nigel Owens (WRU).