I can't take total credit for this solution as somewhere I saw something similar on-line but for the life of me I can no longer find it.
If you are like me you jack your tractor up to clean the deck and swap blades from underneath. That means you are working upside down and gravity is working against you.
One of the problems I have had is how to keep the blade from turning while you torque the retaining bolt. A while back I had bought one of those fancy blade holders shown below. It was around $20 and while it does work on the outer blades it is very cumbersome to attach and detach and it's nearly impossible to use on the center blade (it does work quite nicely however on a push mower).
I've also tried using a block of wood, which does work as long as the blade is pressing against it but as soon as you relax tension to reposition the torque wrench the block of wood has a tendency to fall out and do its best to wack you in the face. If you remove the deck and stand it on end or flip it upside down the block of wood works great.
Which brings me to the solution below. I cut a chunk of 2-1/4 inch exhaust pipe and then squished the end so it would just slide over the width of the blade. The length is such that it is easy to slide on to the blade but then when the blade tries to rotate the other end of the pipe section will contact one of the deck baffle walls and prevent the blade from turning. It works amazing, does not fall off and hit you in the face, and is extremely inexpensive as a 27-inch chuck of pipe is less than $6.00.