Blues, who made three changes to the squad as they called up Harry Davies, Rhys Shellard and Jevon Groves, qualified as Welsh champions two weeks earlier.
And they served up a cracker in the opening game of the night and quarter-finals with Leicester Tigers as they battled back with two tries in the final 50 seconds to snatch a 19-14 win.
Tigers had looked destined for a place in the final four with a nine-point lead with less than one minute remaining.
But a tries from star performer Dan Fish and Tom Williams – with time up – saw Blues book a place in the Cup semi-final in the most dramatic of fashion.
The weather slowly turned for the worse in the English capital as torrential rain swept in for the last four showdown with Gloucester – but the teams still served up a wet and wonderful encounter.
Fish was again the star man as he grabbed a brace of tries to take his tally to four on the night – and six in total in the Premiership 7s Series.
However, it was Gloucester, with two late tries, that took a 24-14 win and ended the night for Blues who left with heads held high after an impressive night of 7s rugby.
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