To be honest, Zone-Blitzes are classified differently by all teams. Traditionally, a Zone-Blitz would have a Cov. 2 or 3 Zone behind a 7 Man Front. One of the Linemen would drop off into an area that the Blitzing LB has vacated.
In today's more sophisticated schemes, though, Zone-Blitzes have expanded too. Several teams will man up across the board, blitz a safety and LB, and drop a DE into the Hook/Curl area or Flats. It's still a Zone Blitz even though there is a Cov. 0 look to it.
Something becoming more popular with athletic 4-3 teams these days is having a Zone-Man Switch Blitz where you have your DE man up on the TE or RB, blitz the LB and still play a Zone behind it. This occurs a lot when a team is getting beat with quick hits to the Backs or TE's (most comming in HS or college).