Contrary to its name, a Blank Check isn't really blank.

On a real blank check, the only field filled in is the signature. If you sign your checks in advance, this is a bad thing.

If someone steals one of your checks signed by you, they can fill in the rest of the fields to their heart's content, spending lots of your money.

They're convenient if you give one to someone you trust. If you don't know how much something is going to be, you can fill out the date, to, and signature fields, and tell the person to fill in the amount when they know. I would never do this, though lots of people do.