OS X Update 10.10.3 Boots To Black Screen

On my iMac (latest 27" retina) only and not on any of my 5 other macs (none of which are iMacs), the latest OS update after download from the app store and subsequent reboot, just stays at the black screen with only my mouse pointer visible - till the computer goes to sleep - and then on manual restart, same black screen after I have logged in. This computer has no non-apple components that might cause any kext signing or other compatibility issues and neither are any external peripherals plugged in except for the Bluetooth keyboard and mouse it came with. All of the macs are filevault encrypted but this is the only one failing to successfully update (all are running Yosemite and two were bought at the same time as this machine)

I know that if I first, recover this mac from Time Machine and second, turn off filevault, then I will successfully be able to install the update. And I know, that I can simply turn filevault back on after the successful updte. But I really feel that installing an update should be a tad more user friendly and less time sapping than all this. Any pointers?