It seems like it ought to be easy to fool the user's unconscious processes that determine fixation while still making clear at a conscious level what is and is

It seems like it ought to be easy to fool the user's unconscious processes that determine fixation while still making clear at a conscious level what is and is not an ad. For example, match the color scheme and typography of ads with the rest of the site, but clearly label them as "sponsored links". While it's possible that you might annoy users this way, it would be a real stretch to call it unethical.