The chance of dropping Uniques increases with each Torment tier, so yourbest chances are in Torment 4.
The Razorplate is no different, and can drop from all of these sources and more.
However, if you want to target-farm this Unique, you do have one option.
you could get Exquisite Blood from the following activities.
What Does Razorplate Do?
The Razorplate is a Unique chest armor that, instead of normal stats,only provides Thorns.
This means that every stat slot instead increases your total Thorns value.
Use Resplendent Sparks to craft any Mythic Unique you desire.