The dealer and Blazer are correct. There is more stress on the axles with a lift. In fact, I'll bet if you go for a spirited run on the stock setup, take a heat gun to the shaft, then do the same run (with the same ambient temps) after the lift, and take a temp reading, it will be appreciably higher.
Kind of like using universal joints on a ratchet. As the angle increases, torsional stress and friction increase, and a universal joint used at more extreme angles is going to fail sooner than one used at far shallower angles.
Also along those same lines is your steering angle. While you may not be able to control the ride heigh axis of the CV joints, more often than not you can control the steering angle. Making a conscious effort to not give it too much torque (the skinny pedal) when your wheel is locked full left or right will go a long way to preserve your CV joints. How aggressive you are in tackling a hill climb is yet another factor. If your front wheels pop off the ground and land while you're on the gas, you're far more likely to snap them.
I have the BP lift and got it knowing I'll likely have to replace CV joints earlier than I would need to otherwise. The trade-off is worth it for me, I bought the Teryx as an adventure vehicle to take places I'm unwilling (due to potential damage) or unable (due to size) to take my truck to. I check the boots for tears and surrounding areas for any grease that may have leaked out and make sure the shaft itself doesn't have any excessive play after each outing.