devel/b2 requires arrow<0.12.1, but 0.13.1 is now the current version in ports. Error occurs with both b2-2.7 and b2-3.6.

This version restriction was apparently to keep supporting Python 2.6, a pull request has been created but still not merged:
https://github.com/Backblaze/B2_Command_Line_Tool/issues/551

I haven't tested it yet, but this patch should fix the issue:
https://github.com/sww1235/B2_Command_Line_Tool/commit/5c82bd6ca99a5e161ed0939a30ffb4a4128404b1

% b2
Traceback (most recent call last):
  File "/usr/local/bin/b2", line 6, in <module>
    from pkg_resources import load_entry_point
  File "/usr/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 3191, in <module>
    @_call_aside
  File "/usr/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 3175, in _call_aside
    f(*args, **kwargs)
  File "/usr/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 3204, in _initialize_master_working_set
    working_set = WorkingSet._build_master()
  File "/usr/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 585, in _build_master
    return cls._build_from_requirements(__requires__)
  File "/usr/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 598, in _build_from_requirements
    dists = ws.resolve(reqs, Environment())
  File "/usr/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 786, in resolve
    raise DistributionNotFound(req, requirers)
pkg_resources.DistributionNotFound: The 'arrow<0.12.1,>=0.8.0' distribution was not found and is required by b2