I havn't checked the method (and it currently doesn't happen for me, btw. - I'm logged in on GMail, but logged out on YouTube). But third-party cookies aren't necessarily what is happening here. There are other ways, similar to the way OpenID authentication works, or the way Wikipedia (probably) logs you into all of Wikimedia.
youtube.com embeds an image
google.com/authenticate?id=uniqueid&domain=youtube.com, then gets a Google-internal callback with your Google identity, to log you in.
Or the other way round: when you log into Google, they load an image from YouTube - say
youtube.com/authenticate?id=uniqueid&source=google.com, then do an internal callback with the unqiue session ID to retrieve your authentication data and set another regular cookie on the
I don't think you will be able to block this easily, not unless installing a proxy filter that e.g. disables all cookies from youtube.com; using "incognito" mode, or essentially rejecting all cookies from youtube.com in the browser (blacklist).