I'm running El captain public beta but I think this also applies to macs running Yosemite. I really like the look of the reduce transparency + increase contrast in Yosemite/El Captain. Unfortunately those options also force the lock screen background to be a boring grey colour instead of the blurred version of my desktop background :(
I am aware that you can change the lock/login screen background by navigating to /Library/Caches and changing a file. I checked and the file is as it should be - the blurred version of my desktop background.
Is there a way to set the lock screen background to something other than the grey colour while still keeping reduce transparency and increase contrast on? Perhaps something to do with changing system files?
EDIT: I did some research and it seams like this behaviour is unique to El Captain. Yosemite has the desired behaviour of the lock screen being the blurred background even with reduce transparency and increased contrast turned on