I don't know if the PM's just get discarded. All I know is, if you write one and click submit.... a message will come up saying the person isn't accepting PM's. When you have a member on ignore you can see their name and the fact they posted but not the post - there's is the option to look if you want. I'm a curious person and can't resist the urge to look lol.

I should also point out that there's an option on settings where member can choose to receive PM's from all or JUST their contacts, administration and moderators. This is different that being on someones ignore list.