bcrypt isn't reversible (with respect to the password). The point is that Blowfish(key1, "message") is not at all similar to Blowfish(key2, "message"). This is called resistance to related-key attacks (look this up for a formal definition), and is an explicit design goal of every real-world block cipher.

bcrypt is just Blowfish(key-derived-from-salt-and-password, "OrpheanBeholderScryDoubt"), so it inherits this property.