I've just seen this from our intrepid cockrider, but I'm pretty sure it only says that when they have blocked you or their account is locked down, not the other way around. If you have blocked somebody it says 'this tweet is from an account you blocked' with the option to view.
Once again <insert Jimmy Nail gif>
ETA: Squigs have noted this.
Once again <insert Jimmy Nail gif>
ETA: Squigs have noted this.