The Russian Ka-50 Black Shark attack helicopter has an ejection seat for the pilot. According to Wikipedia, the rotor blades and canopy are blown away by explosive charges before the ejection seat is triggered.
Part of the reason other attack helicopters don't have them is that ejection seats in helicopters add complexity and weight for not that much of an increase in survivability, considering the usual flight profiles that these helicopters fly.
It makes more sense to focus on improving the survivability of crashes through measures such as collapsing seats and landing gear designed to absorb impact forces. With these types of improvements the crew of modern helicopters can survive even severe accidents without serious injury.