I wouldn't have thought the blight would have reduced it. Look at the Agonians. They have high resistances to disease and poisons which they gained because of there harsh enviroment.
It would be more likely that such an inhospitable enviroment with the blight wafting about ,would most likely strengthen there resistance to disease(survival of the fittest and all). But thats just my 2 cents.
Yeah, if you're a group of idiots who decide to settle in the blightest regions of Vvardenfell naked. Survival of the smartest, don't be in areas where blight is a problem or avoid blighted animals and plants.
As I said, high elven resistance to disease is probably born from the High Elven eugenics programs intended to create a perfect individual worthy to be called descendant of Auri-El. Provided all those rumors about High Elves are true.