Opponent Block meter full with 6 cards...

You forgot Monkey Smuggler. The only two cards that can prematurely fill your block meter (not being attacked) are Body Gourd and Monkey Smuggler.

I do not understand the issue. The OP saw an instance where a player had a full block meter and only 6 cards? Or is the unusual part the OP continued to take damage and not block with only 6 cards in hand? Because for the latter I would guess Bulls Eye too.