They also ended a run of eight successive defeats to the Scarlets with centre Allen a star performer.
The Wales international picked up a shoulder injury against Argentina the last time he played at the home of Welsh rugby.
And the 21-year-old admits it has been tough coming back from the complaint.
"It was difficult psychologically coming back after shoulder surgery, but it gets easier with every game. Getting a game under my belt at the Millennium Stadium once again was a big boost and I'm now two from two there," Allen told the Blues website.
"It was good to be back playing at the Millennium Stadium and great that we were able to make it three wins on the bounce. It was certainly good to get another win.
"It was great to be able to play against Jonathan Davies. It always brings out the best in me when I play against the top players in the game.
"I'm pretty fresh at the moment, but I was blowing quite a bit by the end of what was a fast and furious game. I'm still hoping to get a call for the Wales tour to South Africa this summer."
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