The Blank Voucher is the most confusing of the many unique consumables and collectibles in the roguelike Balatro. I'll walk you through the process of obtaining the Blank Voucher and using it to make your next run successful.
Balatro: How to Unlock the Secret Voucher
Redeeming the voucher, as the description states, does nothing for you in a run. However, redeeming the voucher does help you get the Anti-Matter Voucher, which is extremely useful for fighting the Blinds.
Related: Balatro Joker Tier List
In the Balatro main menu, tap on the green "Collection" button on the right. From there, press the "Vouchers" category on the left. If you have purchased the blank voucher before, you will see it on the third page in the bottom right corner. Move your mouse or controller to the locked space next to it, and you will get the hint: "Redeem blank voucher 10 times in total."