It's not just ads, but anything that visually sets itself apart from the rest of the content on the page. This includes sidebars next to articles and even headings at the top of the screen, if they look too different. It's called banner blindness https://en.wikipedia.org/wiki/Banner_blindness I had to deal with it in a product where the current location was shown in a purple tab at the top of the screen. Users would consistently get lost because, even when looking for location cues, they never read the text.