When I was playing on the DMU, a player named BurgerMod3 said that he found an item called the collector's black box. Then he says that his hotbar items disappeared.

I went into a creative singleplayer world, and found that when you have it, items in your hotbar disappear when you select them,is this supposed to happen?

Yes because nobody except John(1WTC) and Adam(Kaneras) can use it. Its tied to their usernames.

But what do they actually use it for? 

*and Hughpugh (John's brother)

They can blow stuff up with them. It's a Team Fortress 2 weapon I believe

Cool!!!! I want to be able to use it in my single player worlds. ;-;

 Sorry but its a John thing, Like the Sword of Dashing is RedDash's and the Sooby Sword for Sub, and the other weapons that some staff have.

Okay, I understand.

 Try the "notarocketlauncher" in 1.7.10 if you want.

It's the next best thing

Okay! Thanks!! :D