Some encryption algorithms have weak keys that cause the encrypted data to be analyzed and potentially more easily. I don't believe that Blowfish has any proven weak keys. DES, on the other hand, has some known weak keys. With blowfish a short or predictable key is not a weak key in that respect and your data is just as well encrypted as data encrypted with a long/statistically random key.

The weakness is that if your keys are bad that they may be guessable and that you may be only using a small section of the whole keyspace. The diffrence between the problems of "certain keys cause poorly encrypted data" and "guessible keys/small keyspace utilization" are subtle but important.