The nice thing about using objects here is that you wouldn't have to hardcode the block list file. You could pass it to the constructor. As well, you could keep the contents of the file in the object and you wouldn't need to keep reading and writing the file.
Of course, if you want to have multiple processes consulting the file, you'd want to check if it was updated since you last read it, so keep a timestamp around as object data as well. Then you could open, lock, and write the file only if you changed the status of an IP address.