I wouldn't really say it's "secret" and I could say the same thing about the command block or structure block or structure void or puzzle block as none of these show up in the creative inventory. But that being said and have quite a bit of experience in commands myself I can see how this could be considered a "secret" block I suppose. Also, I can see how this post could be quite nice if I didn't know about it already. :)

D:)

yeh

I have spells that surround player in barrier blocks, cakes, lava, end portal blocks.
The blocks disappear after 10 sec. A barrier block maze is a nightmare to navigate.

Very handy for stopping players moving into restricted areas or falling to death at public warps.

Worldguard. blacklist.txt A Creative world anti barrier block?
[​BARRIER] on-acquire=deny,tell message=Sorry! %s! are staff only in Creative world.
ignore-groups=ethereal,benefactor,modi,modii,admini,adminii,dev,adminiii,maker,op

Other players can't see the barrier unless if that player is holding a barrier block.