Are collisions possible on the output of Blowfish?
No, you cannot, unless you have repeated messages, or use different keys. A block cipher such as Blowfish is a keyed random permutation or bijective mapping.

That means for each key any plaintext block is mapped to exactly one ciphertext block during encryption, and the ciphertext block is mapped to that particular plaintext block during decryption.

For different keys there are different mappings. So if you mix keys you may get a collision.