After receiving the request for a medical evacuation mission, the crew checked weather with an FAA FSS to determine if the mission would be accepted. Ground clutter on the weather radar prevented a complete weather brief for enroute mountainous areas. The pilot got weather for his departure point. The pilot told FSS '...I think I can meander my way through the mountains on that one.' The pilot accepted the mission. After takeoff the pilot was told by FAA departure control there was 'weather' on his route of flight 'but the intensity is unknown.' Radar service was terminated and the crew continued. The surviving crewmember stated that as the flight progressed, everything got 'black.' He said he could not see outside the aircraft. He said about 5 minutes before the crash, the pilot told the crew '...we are going inadvertent.' Meaning, they were going into the clouds and he would be flying by instruments. The survivor said he heard the 'altitude buzzer,' followed by the impact.