any ideas why a word whose etymology is "flat" or "smooth" comes to mean "cliff" or "grove of trees"?
A bluff has a smooth or flat face. The grove of trees could well have a face that is reminiscent of a bluff.
So, its bark is worse than its bight?
Compliments on your Dutch! From blaf to bark is mistery too, but I know by now language is just like people. No logic.