Agree, avoid disc-specific racks altogether unless your brake layout dictates you have to have one. Hydraulic brakes tend to be more compact than cable ones and can often be fitted in without any special provision. Using a bigger disc sometimes helps as well.
The Blackburn is awful, and to be avoided at all costs.
Not a comment on their racks in general, because in general they are excellent and very strong - but their disc-specific model is just a disaster. It mounts onto the wheel spindle using an over-length QR replacement. Makes rear wheel removal in case of punctures, a real pain, and doubly so if you have luggage on.
It's also possible if you're lucky, to fit a normal 2-legged Tubus rack 'around' the brake, like this:
These racks are tougher than they look and for anything short of full-on camping loads, they are the bees knees - although completely useless for day rides owing to the narrow top platform.