Don't let anyone tell you you can not get blade ice at -40. Although Malabo is right, USUALLY the air is too dry, that may not be the case over open water.
Also, the most dangerous temp for a helicopter is approx -7 and below depending on blade length and RRPM. I used to have the formula that would determine blade temps in relation to length and RPM, but I burned it along with my parka when I left the arctic
As the temp decreases from zero ice will start to build on the head and move outward. You will not notice too much of a problem with Q until it starts to affect the outer 1/3, again around -7. Of course if you need to autorotate the inner 1/3 is pretty important as well
