I just noticed that it also returns a 404... is your index.php file in the /blog directory? I'm just wondering if something went wrong with the upgrade.
Also, what version were you using before the upgrade?
How about plugins? Did you disable all of them before upgrading? If not, since you said you can access the panel area, maybe try disabling all plugins and see if that helps. If so, enable them one-by-one until you hit the one that causes the problem.
Otherwise, support should be able to figure it out. If your setup isn't too customized/modded, you could just delete the directory and re-install it. It should just connect right back to the DB when you add the config info... but someone smarter than me would probably back up the DB first, just to be safe.
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