"Well, if I understand correctly, since an ads placement depends on its CTR and CPC it's possible that blocking *bay (for instance) might bring up an ad that has a lower CTR but higher CPC. "
If you block an ad that adsense is displaying, you will get an ad with a lower earnings per click than the ad you blocked. Adsense displays ads with the highest costing ads per click and then down the list. That is the point of bidding for keywords, if you bid higher, you have a better chance of being picked for selected keywords.
If you block an ad that you see on your page, adsense assumes that the ad is showing is a good match, and has one of the top 4 highest cost of all ads that fit the criteria of your page. If you block the ad, your earnings per click for the ad that will replace it will be lower, maybe a penny lower, maybe a dollar lower.
On the other hand if you block an ad that has little to do with your page, and get in a new ad that has a better click through rate, the tradeoff of banning the higher paying ad with the increaes in CTR for the new ad might yield more earnings.
Blocking an ad will result in a new ad that has a lower CPC, unless new advertisers come along at the same time you are doing that and bid more than the ad you blocked. Your CTR MAY go up if the ad is more relevant to your pages, and visitors find it more attractive to click on.