Another one:
The barrel's item indicating image is "dark". This applies to "locked" and "not-locked" barrels.
Good thing is that when a barrel is initialized with an item it is locked to that item. Makes sense.

Originally posted by @Knito58 in #41 (comment)

What I'm aware of just now:
Also the barrels in an inventory are already shown "dark". I believe they used to be brighter before the breaking update. Maybe that darkness is already applied to the barrel item before it is placed into the world as a block.