Big-O Encryption Algorithm
It is $O(1)$ time and space because the Blowfish block cipher has a constant bound on the key size (≤448 bits) and a constant block size (64 bits) and those are the only inputs—there are no variable-size inputs.

If you apply Blowfish in a composition like CBC then it is expanded to work on $n$ blocks by applying the Blowfish block cipher $n$ times, so it costs $O(n)$ time and $O(1)$ space.



P.S.  FYI, Blowfish is obsolete, and has been for decades since the standardization of AES in the late '90s.  There is no reason to use it today, and strong reasons not to use it or any other 64-bit block cipher.