A disappointing opening half from Blues saw the home side – the only unbeaten side left in the league after six rounds of action - build up a commanding 19-3 lead in Swansea.
Ospreys dominated possession and fly half Dan Biggar was deadly accurate as he made Blues pay for any indiscipline with four penalty kicks.
The visitors also found themselves the wrong side of referee Leighton Hodges and as the penalty count mounted, and Sam Warburton was yellow carded, the Ospreys made the extra man count with full back Dan Evans touching down.
A second half close range score from Rhys Webb ended the game as a contest before Blues showed plenty of spirit late on with tries from Lloyd Williams and Kristian Dacey in the final fine minutes.
A fired up crowd roared the game into life and it was the hosts that settled the quicker with Biggar giving them a second minute lead as Blues were pinged at the breakdown.
Tempers flared at the very first scrum of the game, as the home side were awarded a penalty as the front row went down.
Ospreys mounted a period of sustained pressure before another penalty was awarded by referee Hodges for offside and Biggar slotted another easy kick front directly in front of the posts.
The game was being played totally in the visitors half in the opening ten minutes until hooker Matthew Rees pounced on a stray lineout throw from his opposite number and Blues looked to mount an attack from deep.
However, it was only a temporary reprieve as more and more waves of attack came from the hosts and, when Hodges awarded his third home penalty in the opening quarter despite Rees looking to win the ball, it was Biggar who did the rest.
The Ospreys were looking to run the ball at every opportunity with full back Evans attacking the line at pace and lively scrum half Rhys Webb looking to set Lloyd Peers into space after making a dart around the base.
Referee Hodges lost his patience with Blues at the breakdown, with Warburton taking the yellow card and Biggar making it 12-0 after the first quarter.
Despite being man a down, Blues suddenly came alive to enjoy their best period of the first half as Peers was stripped of the ball by Josh Navidi.
Cory Allen cleverly kicked in behind into space to turn full back Evans and Rees and front row partner Adam Jones took ball on before a penalty allowed Patchell to crunch over a kick from halfway.
It was Ospreys, though, with the extra man and they looked to make that count as slick hands nearly sent right wing Jeff Hassler in.
However, there was no mistake less than a minute later as play went left with pace and, after Josh Matavesi’s break took him close, the ball was rattled into the hands of full back Evans who stepped back inside the last man and dived over the whitewash.
Blues needed to regroup before the break and looked to their kicking game to win them possession and territory. But it was the Ospreys who menacingly finished the first half stronger as Eli Walker had a try chalked off for a knock-on in the build-up.
Great work from Gavin Evans at the start of the second half saw him race back to collect Rhys Webb’s kick and tear past tackles and out the 22. But another strong kick from Evans into the opposite corner saw Blues have to immediately play from their own line.
A penalty for dragging down a driving lineout saw Ospreys turn down a kick at the posts and instead go for the corner. But Blues held firm as the home side spilled ball down the short side.
Scrum half Lloyd Williams and Number Eight Manoa Vosawai were early second half replacements for Blues.
But the pressure continued to flow from Ospreys with Jeff Hassler making a break and feeding big prop Dmitri Arhip on his shoulder.
Moments later the play again found the hands of strong running wing Hassler and when he was tackled short the ball was picked up by Webb who scored. Biggar’s excellent conversion made it 26-3 with over half an hour to play.
Blues produced their best period of pressure in the game as they recycled and strung together phases to make big yards. Williams darted into space and fed the marauding Vosawai and Patchell, but when play broke down the penalty went against Hammett’s men.
Ospreys blew a definite third try as the elusive Hassler broke in midfield and only had to find a wide open Andrew Bishop, but the pass went behind him and he spilled with the line beckoning.
Vosawai was carrying strongly and Warburton and Walsh found gaps before only a tap tackle stopped Allen as he looked to break away.
Ospreys looked to strike once again as they twice kicked into the corner in the dying ten minutes. The first driving lineout was only just halted before a stray lineout throw was collected by Warburton who hacked clear.
Blues looked to mount a late charge in the final five minutes, but a quick tap penalty from Gareth Davies came to nothing as the ball was spilled short of the line.
Minutes later Blues finally had some reward from the game as Thomas broke a tackle from Matavesi and fed Williams to slide over.
The home side had Morgan Allen yellow carded with time up before Kristian Dacey drove for the line and after a lengthy TMO decision the try was awarded.
Timeline:
02m Biggar p 3-0
07m Biggar p 6-0
11m Biggar p 9-0
22m Biggar p 12-0
26m Patchell p 12-3
29m Evans t Biggar c 19-3
52m Webb t Biggar c 26-3
78m Williams t 26-8
80m Dacey t Patchell c 26-15
Cardiff Blues: 15 Adam Thomas, 14 Alex Cuthbert, 13 Cory Allen, 12 Gavin Evans (Davies 17-25, 59), 11 Geraint Walsh, 10 Rhys Patchell, 9 Lewis Jones (Williams 45), 1 Gethin Jenkins (Hobbs 59), 2 Matthew Rees (Dacey 59), 3 Adam Jones (Andrews 72), 4 Jarrad Hoeata (Cook 67), 5 Filo Paulo, 6 Josh Turnbull, 7 Sam Warburton, 8 Josh Navidi (Vosawai 49)
Replacements: 16 Kristian Dacey, 17 Sam Hobbs, 18 Scott Andrews, 19 Macauley Cook, 20 Manoa Vosawai, 21 Lloyd Williams, 22 Gareth Davies, 23 Dan Fish