Despite whatever specific services they may offer, Google is an advertising company, and that's where they get the vast bulk of their money from. All of their websites are built around using ads, and getting feedback through various services, scripts, and domains is a big part of all their sites, even if you don't visibly see it.
Over the years they've purchased various ad related companies, such as doubleclick, so it stands to reason that if you block domains that they own, or are related to advertising that you will see an impact on various Google services. It's complicated by the fact that Google uses so many different domains on their various sites, and seems to change them periodically. I haven't checked out the DNS entries for the various domains, but it's entirely possibly that some of them "point" to others as well, which could further complicate things.
Is your only purpose in using OpenDNS to block ads, and that's why you aren't blocking any categories, and instead only blocking those 3 domains?
