Mine can blow their noses, but they really hate doing it - make a huge fuss and prefer to sniff, and when they do blow, it's normally so gently it doesn't do an awful lot... So don't assume knowing how to blow your nose equates with being sensible enough to do it!
On a connected note, anyone else had trouble teaching their child how to spit? My eldest finally worked this out, but my younger one still swallows his toothpaste and then makes a funny noise, which he seems to think is spitting, even though nothing comes out...