James, 20, said: "I was gutted to miss the Six Nations but it was great to be back and scoring tries. I felt strong and that was one of my best games for the Blues.
"The win may be too late for the Magners League title, but we've got so much to play for against Toulouse next week. That's the big one."
Nick Robinson also returned from a long-term groin injury with a 20-minute appearance off the bench. Jamie Roberts scored the Blues' third try.