Blundstone 500s use Polyurethane (PU) soles, which are meant to be used. They grow brittle when not worn for a while, causing cracking, and humidity causes them to crumble. The only way to avoid this is to wear them often (once a week or so) and they'll last a long time. Otherwise, pay a $20-$30 more, and buy the 550s, which use a TPU sole and resist this problem much better. But you still have to wear them at least a couple of times a month. If you expect to keep them in your closet for months at a time, there is a good chance one day within a year you'll find the soles have turned to crumbly soft clay. If you wear them often, this problem probably won't affect you.
10 years ago they used a different kind of rubber for their soles, tough as nails and no problem with cracking/crumbling. Someone in the company decided to save some money, I guess, and started using these cheaper soles - a bad decision, I reckon, but they haven't gone back.
If you like this style of boot but don't want to gamble with this problem, consider a pair of Rossi boots - very similar look, more expensive, but no problem with deterioration of the soles.