I expirienced the same issue on my 928F - black screen and only blue led.
Could not turn off phone or go to recovery by any key combination. After a lot of attempts, managed to go into download mode, than vol down to cancel and go to recovery (vol down-home-power).
After that wipe all, instal ROM, wipe dalvik and cash, instal kernel, wipe dalvik and cash and now V5 onboard!
what you have choose ? magisk or supersu ?