In case of int() the name of it's first argument was documented, in both the module documentation, and in interactive help. But the documented name of the first blake2b() argument was "data", and it never worked. Since help() was not worked for blake2b, the user had weak chance to know that the actual name is "string". Thus there is much less chance of breaking the user code by making this parameter a positional-only than in case of int().

But I think Christian has other concerns.