Former Cardiff coach Dai Young took his London Wasps to his old patch in the LV= Cup, but it was current incumbent Phil Davies who had the last laugh in a thrilling 22-17 win for the Blues.
His decision to introduce experienced fly-half Ceri Sweeney as a replacement for the misfiring Gareth Davies was rewarded as the former international grabbed the winning try as the Blues came from behind to end a six-game losing streak.
Dan Fish and Tom James also crossed for the hosts, with Elliot Daly going over for the only Wasps try.
Wasps fly-half Tommy Bell saw an early penalty blown wide on the breeze, before the home side took the lead with a superbly-worked try from a scrum. Full-back Fish ran wide to take a long pass from scrum-half Lewis Jones, straightened, using his winger as a foil, and raced through to the posts for Davies to convert.
Bell adjusted his sights to slot over two penalties, while Davies missed a chance to respond before the Londoners went ahead thanks to the efforts of two players who each passed their 20th birthday in the past month.
No 8 Billy Vunipola took the ball from a quickly-taken free-kick in midfield and found full-back Daly on his shoulder to accept the pass and reach the corner, stretching over the line despite a desperate tackle by Fish. The conversion went astray, but Bell made up for it with another penalty from the final kick before the interval.
Davies opened the second-half scoring with a penalty, but missed a straightforward repeat, prompting groans from the 7,046 crowd and his disappearance, Sweeney arriving just in time to watch Bell show how it should be done and add another three points.
The Blues hit back immediately, however, an excellent crossfield move creating space for Tom James to speed in by the flag, although Sweeney could not convert from the touchline.
But the substitute fly-half made amends with a brilliant individual try, barging through three attempted tackles to touch down under the crossbar, giving himself a simple job to add the extras.
It looked as though flying Wasps wing Christian Wade had rescued the visitors when he chased a kick ahead in the dying minutes and seemed to have been impeded in the act of scoring. English referee Luke Pearce ignored claims for a penalty try, ruling that Wade had knocked on.