Did this started happening after yesterday's Lightroom update CC2015.3 ?
Could you please check if you are on CC2015.2 or CC2015.3 ?
Go to Help menu> System info
If you are on CC2015.2, would suggest you to update to the most recent version.
The blue overlay indicates shadow clipping. Press J to turn it off.
A red overlay indicates highlight clipping.
Thank you for your help and suggestions - it was before the update - though I have updated LR today - and I'm also hoping that the recent issue of photos when loaded were blacked out and required a reset and Auto Tone - on the Quick Develop button has been resolved?!
Thank you Per Berntsen - that was exactly the issue! I must have inadvertently clipped the "J" and turned it on! Phew! Normality restored.... for now!