Blokket is simply a pouch made of silver mesh embedded in a nylon material, and that is the reason it is able to block electromagnetic fields and data transfers.
It's capable of enclosing RFID chipped ID cards, credit cards, and passports, but is also able to hold most phones up to about 8" x 5.75", which effectively takes the phone offline during the time it is in the pouch. That prevents NFC (Near Field Communications) from carrying out data transfers, including those related to financial transactions or passwords.
When it comes to smartphones, a better alternative is probably to turn off the phone, or at least activate airplane mode; but that just doesn't work with data storage devices that are actually powered and activated by the electromagnetic field that initiates communications with a passport or credit card, for example.
Blokket claims to block 99.7% of radio frequencies. It is amazingly low-tech in its construction, while carrying out its high-tech mission of keeping a few sensitive items "off-the-radar" so to speak, and brings to mind the Stealth Tablet Envelope.
The Blokket pouch is hand-washable, made in the USA, and available for about $25 at ThinkGeek.com.