I find there are two ways to go with blending. Contrasting - so they stand out more on the page as ads eg using red ads on a blue page. Blende - similar/same colors as the layout so they blend in and look like part of the page (sometimes generates accidental, false clicks)
blending will gives you more clicks but not very much from your regular visitors, it would work more better if you use a link color rotator in your blending ad format.
As a general rule: contrasting works better when you have a dark background page. When you have a light background you should blend your ads.