If you're on one engine and you can't get the gear down, you're having a really bad day but, when you think about it, blue line is not really an issue. Emergency gear extension will be needed on the way down rather than in the climb. Gear extension speed will certainly be above min asymmetric control speed and, even then, there should be no need to have lots of power on when you lower the gear. Flying the aircraft below blue line is not a problem as long as you've got a bit of height to accelerate back to it if you need to climb.