A kicking duel between Dan Biggar and Dan Parks resulted in the first drawn match between the Ospreys and Blues in the Celtic League.
The visitors trailed from the fourth minute until Parks brought the scores level with his second drop-goal of an absorbing match with 12 minutes left.
Quality attacking play was rare, with poor discipline from both sides giving the fly-halves their chance to shine.
James Hook and Biggar missed late drop-goal attempts to snatch a home win.
The result lifts the Ospreys above Leinster into third place in the table, while the Blues leapfrog the Scarlets into fifth as the race to finish in the top-four play-off places hots up.
The Ospreys enjoyed the upper hand in the opening exchanges, moving the ball swiftly through the hands, hitting the rucks with real intensity and demolishing the Blues pack in the first few scrums.
Penalties inevitable followed from the early pressure, and Biggar confidently struck over his first three attempts to open a 9-0 lead after 22 minutes.
The balance of power then suddenly shifted to the Blues, and Parks made amends for his earlier miss at goal by stroking over a drop-goal and a penalty in the space of four minutes to cut the deficit.
But the Blues comeback was halted in its tracks when Bradley Davies was sent to the sin bin for an offside offence deep in his own half. Biggar collected another three points from the resulting penalty.
Parks hit back immediately with his second penalty, but the Ospreys quickly re-established their six-point lead when Biggar slotted over a drop-goal with the last kick of the first half.
Biggar's poor pass in a promising attacking position resulted in the first score after the break when Hook had to reach down to his ankles to collect the ball before settling for a drop-goal.
But the home side's hopes of building on their lead were again ruined by indiscipline, and by now the referee's patience had run thin.
Parks first cut the gap to six with another penalty, and then Ospreys flanker Jerry Collins was shown a yellow card as the home side transgressed once again inside their own 22. Parks' fourth penalty meant the lead was down to three points at 18-15 with half an hour remaining.
The offending at the rucks continued with increased regularity, denying the backs any chance of developing promising attacking platforms.
It was left, therefore, to Biggar and Parks to extend their kicking duel. The Blues fly-half came out on top during this particular passage, landing a penalty and a drop-goal while his opposite number succeeded once and missed his other chance from long range.
As a result, it was anyone's game as the captivating encounter entered the final 10 minutes with the score level at 21-21.
A sense of panic gradually grew into the Ospreys' play as first Bigger and then Hook missed with rushed attempts at another drop-goal.
And it was perhaps fitting that their home side's final chance of developing a match-winning passage of play was ended by another handling error, triggering the last blast from the referee's well-worn whistle.
Ospreys: Fussell, Bowe, Parker, Hook, N. Walker, Biggar, Webb, James, Bennett, A. Jones, A. Jones, Evans, R. Jones, Tipuric, Collins.
Replacements: Byrne for Fussell (47), M. Phillips for Webb (54), Bevington for James (75), Hibbard for Bennett (50), Gough for Evans (45). Not Used: Griffiths, Smith, Bishop.
Pens: Biggar 5. Drop Goals: Biggar, Hook.
Blues: Fish, Halfpenny, Laulala, Roberts, James, Parks, Rees, Filise, G. Williams, Andrews, B. Davies, Tito, Pretorius, M. Williams, Rush.
Replacements: Hobbs for Andrews (53), Paterson for Tito (53), Molitika for Pretorius (70). Not Used: R. Williams, Trevett, L. Williams, Sweeney, Hewitt.
Pens: Parks 5. Drop Goals: Parks 2.
Referee: Christophe Berdos (France).