Meh, I should have known better.
Relying on Windows to update me to latest driver never worked reliably, IIRC.
So I went to AMD site and downloaded latest MD FirePro W5000 drivers. They were no too easy to find so they must be ancient by today's standards.
However, after installation blue rectangles are nowhere to be seen, even when running Debug build and after many trials.
Thanks again for your help and sorry for wasting your time on this.