Blank ads blend in with the background of your webpages. Users can't see blank ads so the user experience isn’t impacted but, as there's nothing to click on, you also won't generate any revenue.
Please be aware that blank ads take up the same amount of space as live ads. For example, let's say you've placed the ad code for a 300x250 medium rectangle between two paragraphs on your page. Until live ads start to appear, there'll be a 300x250 blank space between your two paragraphs.