I think it depends, sometimes ads that are brightly-colored are a turn-off to the visitors, and some will not visit your site anymore because of them. On other cases, ads that are blended perform well, because visitors will think they are a part of your content. While on some other cases, ads that are too blended don't get noticed.
I think the best way would be to experiment with your site, and see which method works best for it.
