But despite scoring three tries before the break, the Blues failed to find their way over the line in the second half as they missed out on a bonus point. Instead Ryan Caldwell and Tommy Bowe went over for Ulster as the Blues' faint hopes of catching leaders Leinster look to have gone out for good.
At least they sent the vast majority of the 8,600 crowd home happy in their last game of the season at the Arms Park.
Blair gave the Blues a perfect start with a penalty after just two minutes, but Richie Rees should have extended that lead in the sixth minute. The scrum half streaked clear after shrugging off his opposite number, but with three men in support outside, Rees opted to kick and the attack came to a shuddering end.
It took two experienced campaigners to provide the clinical edge when Nick Robinson and Williams combined for the game's opening try. Williams launched the attack from a line-out ten yards inside Ulster's half, Robinson burst through before delaying his scoring pass to the Wales flanker, who finished with aplomb past fullback Mark McCrea.
Blair's conversion opened an early ten-point advantage that was reduced to seven by the boot of Niall O'Connor on 20 minutes. But once the Blues found their scoring touch, they blitzed Ulster with two tries before half-time to take a commanding 27-3 at the break.
Tom James was involved in both. The Blues wing streaked through from a scrum in the middle of the park to send Thomas over on 34 minutes. And the up-and-coming flyer was at it again moments later as he and Tom Shanklin stretched Ulster one way, before Williams and Robinson took the attack in the other direction before Deiniol Jones' sscoring off-load to Roberts. Blair converted both tries.
Conditions worsened at half-time and that played into the hands of the Irish who finally got a foot hold up front. Caldwell was shoved over from close range on 54 minutes but was then shown the yellow card moments later as the Blues pressed for a response.
But Ulster withstood the ten-minute spell and when the Blues fumbled possession in midfield, centre Rob Dewey hacked up field for Bowe to score under the posts.
Replacement Dai Flanagan slotted over a late penalty to kill off any thought of a fight back but the fourth try eluded the Blues to the final whistle.