There is, unfortunately, a huge list of possible causes. Since you said you already tried the standard verify and deleting .blob I'll go to some solutions that helped me in the past. Before you start though you should make sure your not in Steam beta because while it generally works, sometimes it causes problems.
A few of my games have had this error (and others) because steam had an update, simply closing and reopening caused the download and patch and my games worked after that.
Another culprit is virus scanners. In particular McAfee real-time scanning interferes with steam from time to time. Simply close steam, turn of real-time scanning (pick the 15 minute option), open steam again and try the game.
I've also had luck with reinstalling games fixing problems. Tell steam to make a backup of Skyrim (Steam > Backup and Restore Games). Once it finishes, go to the steamApps folder and make a copy of the "data" and "saves" folders to somewhere safe like your desktop. These folders contain your custom content and player saves. Now tell steam to delete local content for Skyrim. At this point you might want to restart Steam or the PC altogether. Once back in Steam, go back to the backup/restore menu but this time restore Skyrim and install. Start it up and see if it works. If it does start up, then copy data and saves back to steamApps and you should be ready to go. You may need to reinstall SKSE, I'm not sure if Steam will back it up.
A last resort option (which has worked maybe once for me) is to completely reinstall steam. This is done by deleting everything from the Steam folder except steamApps, Steam.exe, and Steam.ico. Run steam and it will download again.
Hopefully one of these solutions works for you.