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Given a standing ovation on his entrance, Humphreys' last outing ended almost as soon as it had begun after he had been forced to make a number of tackles in the face of Cardiff's bright opening.
The Blues went on to record a bonus point win, with two tries from man of the match Maama Molitika helping guarantee the visitors runners-up spot in the Magners League behind champions Leinster.
On a night when Ulster also bade farewell to Justin Harrison, Tommy Bowe, Neil Best, Roger Wilson and Mark Bartholomeusz, the sight of Humphreys hopping away from the pitch was hardly the way it was meant to finish.
Ulster fell to their second successive home defeat, both reverses resulting in bonus point wins for their opponents.
The severely depleted Blues took the first opportunity that came their way when Roger Wilson was robbed of the ball from an Ulster scrum near their line and flanker Maama Molitika barrelled through some weak Ulster tackling to score under the posts after only four minutes.
Leigh Halfpenny added the extras and the visitors were under way.
Four minutes later and Humphreys' time was up with Ravenhill granting him a second standing ovation as he limped off.
His replacement Paddy Wallace made an immediate impression and with his first touch put Andrew Trimble through a hole in Cardiff's midfield who then offloaded to a charging Bowe.
Bowe was tackled near the line by Halfpenny but the Blues' winger failed to hold him and the Ospreys' winger wriggled over for a score which Wallace failed to convert.
The Blues upped the ante for most of the remainder of the half but failed to take a number of chances that came their way thanks to a combination of sound Ulster defence and lack of composure from the visitors.
Ulster finished the half in the ascendancy after imposing themselves on the game to register their most sustained period of pressure.
With Wallace pulling the strings as Ulster moved the ball at pace, Simon Danielli was put clear by a Wallace pass and brushed past three rather weak tackles for Ulster's second score three minutes into injury time.
Wallace converted the try to give Ulster a 12-7 lead at half-time.
It was a short-lived lead though, as the Blues scored four minutes after the restart when Roger Davies won a kick and chase out of his own half for the touchdown which Halfpenny converted.
Four minutes after that it was replacement second row James Down's turn when more poor Ulster defending saw him sail through the gap. Halfpenny failed to add the extras but the Blues had a 19-12 lead.
Ulster narrowed the margin five minutes after the hour when Wallace sneaked over after the Blues lost control of scrum ball near their line but his failed conversion meant the home side trailed by two points.
Ulster came close to taking the lead when Matt McCullough charged down a Nicky Robinson clearance but it was the Blues who had the final say with Molitika's second try, right at the death, which was converted by Halfpenny.
Ulster (12) 17 Cardiff Blues (7) 26
Ulster: Tries: Bowe, Danielli, P. Wallace. Cons: P. Wallace.
Ulster: Bartholomeusz, Bowe, Trimble, Dewey, Danielli, Humphreys, Boss, Young, R. Best, Court, Harrison, Caldwell,
Ferris, Pollock, Wilson.
Replacements: P. Wallace for Humphreys (8), J. Fitzpatrick for Young (55), N. Best for Pollock (51), McCullough for Wilson (57).
Not Used: Brady, Marshall, McCrea.
Cardiff Blues: Tries: Molitika 2, R. Davies, Down. Cons: Halfpenny 3.
Cardiff Blues: Macleod, R. Davies, Stcherbina, Hewitt, Halfpenny, N. Robinson, Rees, G. Powell, G. Williams, Filise,
B. Davies, Morgan, White, Molitika, Rush.
Replacements: Spice for Rees (59), Yapp for G. Powell (57), R. Thomas for G. Williams (59), Down for Morgan (40),
Shellard for White (69), Lewis for Rush (63).
Not Used: Flanagan.
Att: 9,672
Ref: Andy MacPherson (SRFU).
