I was wondering if it was possible to override the bloom gesture so that it won't exit out of the application, but rather goes back to another scene in my application.
As HoloSheep pointed out on the HoloLens forums
Note however that the Bloom is a system level gesture and not intended for use or interaction at the app level.
I'd still like to use it as sort of a back button in my application, as there is no other good alternative (as far as I could find) for this sort of action.
So you can't override the bloom gesture - but you can disable the Bloom gesture with the Kiosk mode - and then use some other voice command to as your "back button".
"Kiosk mode limits the user's ability to launch new apps or change the running app. When kiosk mode is enabled for HoloLens, the bloom gesture and Cortana are disabled, and placed apps aren't shown in the user's surroundings."