How to blacklist zfs kernel module during boot?

If I'm correct that you installed ZFS via dkms then it looks like you need to remove it via the same tool.

dkms remove [module/module-version] [-k kernel/arch] [--all]

       Removes a module/version or module/version/kernel/arch  combination
       from  the  tree.   If  the  module is currently installed, it first
       uninstalls  it  and  if  applicable,  will  replace  it  with   its
       original_module.   Use  the  --all  option  in  order to remove all
       instances for every kernel at once.

Source: http://manpages.ubuntu.com/manpages/trusty/man8/dkms.8.html

Note: You might wish to avoid using the --all option until you are certain that you have obtained the desired results.