Barrier

Barrier is a block that's not available to be obtained in traditional means, but can occasionally be found in the Auction House.

There used to be a bug that let you obtain it from the Reforging menu. This can also be obtained from an anvil. Putting using the block as normal when you have touchscreen mode on and drag the item into your inventory it will put a barrier where you put it down and give the item back to place it somewhere else in your inventory. When you hold it in your inventory for too long (about 5 - 15 seconds) it gets automatically removed. When held in storage containers (Hoppers, Chests, etc.) they will also be removed.

One safe way to keep barriers as a trophy is to put them in item frames.

A possible reason Barriers are blacklisted is because they are indestructible and invisible, making them overpowered blocks. However, it doesn't have world-bending effects like the Banner, which is why it isn't removed from your inventory immediately.