The former All Blacks legend had a clear run-in for the long-awaited touchdown in the opening minute of the second half.
Cardiff went on to finish with six tries and a vital bonus point and fly-half Nick Robinson claimed 16 points in his 100th match for club and region as the Blues completed a Christmas double, having beaten their other regional rivals the Ospreys last week.
The Dragons made the best possible start silencing the home crowd with a Nathan Brew try after only 21 seconds of play.
But after that it was one-way traffic as the Blues took control. Robinson kicked two penalty goals and converted his own try after a brilliant opportunist effort.
Craig Morgan took full advantage of a huge overlap to claim the Blues' second try and on the brink of half-time Ryan Powell added another.
The Dragons were living off scraps of possession and relied on two Ceri Sweeney penalty goals for their points.
Lomu's try sparked an action-packed second half. Hooker Rhys Thomas's try for the Blues was cancelled out by one from his namesake for the visitors.
Jon Bryant added another for the Dragons before Rhys Williams completed the scoring with the Blues' sixth try 20 minutes from the end.