Well, I would suggest getting the source code to Blowfish so you can identify the functions inside your target. Whether you can crack the app or not depends entirely on how the Blowfish encryption is used. Say for instance the serial number you enter is the key to the encryption, then you're screwed. But if it's like most programs that use it, then they will be using Blowfish to encrypt/decrypt the serial with a hardcoded key. And as you can imagine, it's a piece of cake from there!