Block Defense is an unofficial term by this wiki. Enemies having this term means that they have extremely high defense and regular attacks only deal 1 damage to it. Only a few skills can bypass this.
These are common things that have Block Defense:
Poison can also lower their HP (except for boxes). Critical hits by Members who do not possess Instant Death will deal 2-6 damage. If the enemy has a weakspot, its 3-6.