The blue highlight indicates a moved thread redirection shortcut. These are left in place for a set amount of days (also configurable when moving threads) and eventually expire (or not, if selected not to) - they are a means of letting a thread owner or poster know that a thread has been moved to a new area.
With that said, we have a restricted 'Recycle Bin' area, if you will, where threads are moved and later purged by a configured database cleanup job rather than deleting the item from the database directly or not deleted at all (in the event of a soft delete). This is a private, permissioned sub-forum on this board (only the moderators can access it). We utilize this for ease of recovery (plus we can easily review 'deleted' threads for bettering our moderation skills (I'm kidding of course - I've never been in that particular forum).
One of the moderators (I won't mention any names *ahem*John Clark*ahem*) simply forgot to remove the 'Leave Redirect' option when moving the 'deleted' thread to the Recycle Bin.
Hope that explains it.
*Sheepishly awaiting BBM message from JC*