If the Publisher ( Website Owner ) is selling directly ( No Middle Men / Network ) to the Advertiser and hes transparent , he will use a way to track "real" impressions. Ads that aren't shown to the visitors won't count. ( This is only for CPM based ads ) Publishers know that people use adblocker and can easily detect it , it's not uncommon to replace the blocked ads by an internal image / message asking you to disable it on their website.

But yes , if you buy advertising on some websites , you may pay for impressions that never happened.

Feel free to ask more questions on the subject.