Well you can certainly easily disable the content blocker on a site by site basis in Opera 12, it's an option under the content tab of the site preferences.
I thought that's what the OP was after being able to do.
Hmm. After re-reading the initial post in the light of your comment, perhaps you're right. I took his "content blocked warning" to refer to the site-block alert Opera gives when encountering mixed content on an https site... but he may indeed have referred to the message that arises from generic content blocking, and that does have a site-preference capability under Presto Opera, as you note. I guess only the OP knows for certain which he meant. In any case, I don't believe either kind of situation can be unblocked on a per-site basis in the new Opera.
Just tested it and it works. I remember a time some weeks before where YouTubes HTML5 player didn't work. Maybe it was a problem with YouTube and not Opera. Seems there was a problem with streamcloud. Thank you for the quick answer.
I just revisited the site were a video did not load and it was a flash video indeed, still wondering why that won't load.