Black Vouchers are specific game items that you can use to unlock new characters.  

For example, if you want to change the character that you use to fight zombies, you can tap the icon of the character and choose any other you'd like.

You can get Black vouchers: 

• from red chests when you fight zombies;
• travelling to new locations;
• in the shop;
• watching ad;
• as a reward in Events.