The blanching method seems to work best if you can dump the hot beans into a big bowl of ice cold water. You can even add ice to it. You want to cool them down as quick as possible, running cold water into the bowl if it feels like the water is warming up.
If you can, use the "foodsaver", or vacuum method for bagging them, as it will prevent freezer burn.
I have done green, wax, and scarlet runners this way, and they turn out very flavorful. I'm not familiar with yardlong beans, but I would imagine you don't want them to get overripe before processing them, or they may be tough skinned, and very fibrous.