I recently ordered 2 KK 2.1 HC flight controllers and a display / programmer from Hobby King. When they arrived one of the flight controllers worked fine but the other one just showed a blank screen.
Before contacting Hobby King for a replacement I thought I would do a bit of troubleshooting to see if it was a simple fix.
First I thought I would try to re-flash the firmware. Whenever I connected the USBasp AVR programmer up and powered up the flight controller it screen displayed the normal "SAFE" screen however the menu button did nothing. I tried booting it up again without the USBasp connected: Blank screen again. At least this was a bit of progress.
I then tried flashing the firmware (Stevis 1.9). After flashing the board for the first time I still had the same problem so I tried a second time (just in case...) and this time the board booted OK but it seemed a bit flaky (e.g. sometimes it would boot to the "SAFE" screen and sometimes it would boot to another random menu screen.
I tried flashing the firmware a third time and this time everything seemed stable. Now I just need to fit it to a copter (I'll try an old one first) to make sure that it performs OK in the air.