The 26-year-old had been plying his trade in the Principality Premiership with Pontypridd but will now link with up with the region on a permanent basis as the Blues deal with the long-term absence of the likes of Rhys Patchell and Cory Allen and a minor ankle problem for Patchell's understudy at No10, Gareth Davies.
"Simon joined up with us last year when we were looking for fly-half cover and did brilliantly," said Blues boss Phil Davies. "He was fantastic in the LV=Cup win over Ospreys and the Munster game in the RaboDirect PRO12 was also a very good experience for him as it showed him what is required at this level.
"Simon is an intelligent player and this is a big opportunity for him to be part of our set-up. It's exciting for him and it strengthens our squad at outside half, which is crucially important to us going forward."
Humberstone, who started his career in the Harlequins Academy and won England honours at student level, has signed on until at least 2015.
A former UWIC student, Humberstone is now desperate to develop in a professional environment and give Phil Davies a selection headache, despite Patchell's absence.
"I got a taste about what it was all about back in November in the LV= Cup and, after that whets your appetite, it is all you want to achieve," added Humberstone.
"Being here with Dale McIntosh and Paul John, who I know so well from Pontypridd, makes the move even more enjoyable and I'm determined to make this a success.
"Just being around world class players on a full-time basis improves your game, you have to deal with the intensity just in training and the pressure it puts on your skill set. It makes you a better player.
"It's unfortunate for Rhys at the moment as he is out injured after such a good season, but it gives opportunities for players like me and Gareth Davies to put a marker down for the No10 jersey."