You'll be lucky, and wait a looooooong time. I've handled blueprints for many of the Bristol aircraft, and Concorde too; those went IIRC to the museums at Filton and Brooklands, respectively. Anyone who does still hold blueprints will most likely donate them to museums, which in my mind is the correct course of action. Bear in mind too that the term 'blueprint' encompasses a lot of things from general arrangement drawings to detail design of one single bracket and control rigging diagrams; all were generated by manufacturers back in the day.
I do have blueprints for a couple of US-built aircraft; they came up sporadically on the auction site, but that was maybe 10 to 15 years ago. That sort of stuff seems to be very rare nowadays. However I'd still keep looking - you'll have to wait, but who knows what is still out there to be discovered in grandad's belongings?
In the meantime if you want to view some really nice blueprints, try the National Archives at Kew; they have a special reading room on the second floor for such items, and hold a lot; I've photographed quite a few WW1 drawings this way.