You sign in to Blogger and Youtube with your Google account.
Doubleclick is a good point, though. Of course, you know if you place ads on the site (there's a "Google" in the URL) and you know if you buy the ads. So I guess consumers don't know. Would they care, if they did?
I think that nobody really knows it. It you go to the YouTube home page, there's no mention of Google other than "Add youtube to your google homepage" (might be different in other localization though)
You know. HN readers know.
The rest knows?