James, seemingly ignored in the search for a successor to retired Wales legend Shane Williams, was clearly keen to impress, seeking out opportunities all over the pitch.
But with a youthful Harlequins side finishing strongly, it was the faultless kicking of returning New Zealander Ben Blair which proved the difference as each side crossed for six tries.
With possible qualification long gone and most fans concentrating on Ireland against Wales in the RBS 6 Nations, a meagre crowd of 2,000 turned up for the lunchtime fixture, and at first it seemed that the Blues would also rather have been elsewhere.
After steady Quins pressure, the ball popped out of a maul to flanker Chris York, who found nobody at home in defence and strolled in for a try which was converted by fly-half Rory Clegg.
The Welsh region stirred themselves enough to earn a Blair penalty before carelessness on the left allowed Quins wing Seb Stegmann to steal a loose ball and trot over, Clegg again converting.
The home side finally found some rhythm and James ran onto a short pass from Ceri Sweeney to burst through to the posts. The winger then created an opening from which Luke Hamilton crossed, although the number eight hurt himself in the process.
James then appeared on the right to grab an inside pass and storm away to the corner in the final action of the half.
Blair, in his first home outing after 16 months sidelined with a career-threatening knee injury, converted all three scores.
James completed his hat-trick early in the second period after being sent through a gap by the soft hands of Gavin Henson, a late inclusion at centre following his release by Wales. Blair added the extras.
The Londoners were far from finished, however, hitting back with two tries in quick succession.
Left wing Sam Smith darted over from close range before impressive full-back Ross Chisholm raced 60 yards to touch down in the left-hand corner. Despite the difficult angles, Clegg kicked both conversions.
The Blues regained control with the forwards driving over, young lock Macaulay Cook claiming the try, before James finished off a scything break by Blair, who again improved both.
Injuries left the home team down to 14 men in the closing minutes, which allowed Stegmann and Chisholm to cross for late scores, but Cardiff held on.
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