I recently had to update an HP C7000 series blade chassis. The onboard administrators were both on 1.3 firmware, and a new G7 blade required firmware in the late 2.x line of firmware. I had seen discussions of whether it was safe to upgrade only the OA firmware and not the blades. I contacted HP support, and they said that though while not ideal, you could safely update just the OA firmware.
I first ran the HP Proliant Service Pack ISO: http://h18004.www1.hp.com/products/servers/management/spp/index.html
The HP SUM utility was not able to upgrade the firmware, due to a web API error. I tried several times. The fix that worked for me was to download the last of the Smart Update ISO images, here:
I then used that version of HP SUM to update the C7000 OA to version 9.3. From there, I was able to update the firmware using the Proliant SP disk I linked above. I also used the SP disk to update the ILO's on the blades. I didn't update the firmware on the interconnect devices (in my case, a Cisco switch and a Brocade fibre switch) nor did I update the bios of the blades, although that was certainly a possibility.