If you are shopping the used market, and the ad says bluing is 60%, that means that the finish of your weapon is pretty mucked up. However, the nice thing about bluing is that, with some work to strip the gun down, and the right equipment, you can re-do the process and get a nice, almost perfectly clean, blue-black finish again.
It generally means there is 60% of the finish remaining. In that condition, w/o any pitting it would be a good fixer upper. If it were a 1st generation Colt SAA, it would be considered very good condition (For its age).