Compressor Stall - but the existence of "whiteout" as an optical illusion - even if it existed - still doesn't excuse the pilots!
Every pilot knows - including them - that if you're at only 1500' with a cloud layer above you, ice below you, and rising ground ahead of you - you won't see it. So by descending to this level below cloud and above ice (and not taking any one of several earlier opportunities to abort the descent) Collins had 100% total faith that he was at the entrance to Mc Murdo. And that was simply not justified.
Sure, if at 1500' they were subject to some dastardly optical illusion that made it look like they were in perfect VFR with nothing but flat ice ahead of them; that sealed their fate. But the bad decisions had already been made.