With the insanely cold winter we had i had an incident where my handbrake cable froze and would not release on one caliper on the rear. I could feel it dragging whilst driving home. The pad on that side has had it, completely broken down so im changing them but the disk friction surface is blue from the heat. Is the disk scrap? It does not seem warped, no vibration when braking etc? Im doing a an underside resto so if it needs changing i will get a new disk

 

Disc will be fine, very common for blue spotting to occur where steaks of blue appear on front discs, not a thing you need to worry about, will wear off anyway.
tabetha