Importing a Flask-security instance into my views module breaks my webapp

Short Answer: You are missing blinker library. EDIT: You confirmed that your virtual environment could not find blinker and you re-installed it.

Long Answer:

I think the error is coming from Flask Signals. Look at this code from signals:

signals_available = False
try:
    from blinker import Namespace
    signals_available = True
except ImportError:
    class Namespace(object):
        def signal(self, name, doc=None):
            return _FakeSignal(name, doc)

So I think that the code tries to find the blinker library and in your case, it is not able to import it and hence it tries to use the _FakeSignal class.

The _FakeSignal class does not have a connect_via attribute defined as you can see below

class _FakeSignal(object):
    """If blinker is unavailable, create a fake class with the same
    interface that allows sending of signals but will fail with an
    error on anything else.  Instead of doing anything on send, it
    will just ignore the arguments and do nothing instead.
    """

    def __init__(self, name, doc=None):
        self.name = name
        self.__doc__ = doc
    def _fail(self, *args, **kwargs):
        raise RuntimeError('signalling support is unavailable '
                           'because the blinker library is '
                           'not installed.')
    send = lambda *a, **kw: None
    connect = disconnect = has_receivers_for = receivers_for = \
        temporarily_connected_to = connected_to = _fail
    del _fail

The connect_via attribute that Flask-Security is trying to load is actually provided by the blinker library and since no blinker, no connect_via. Hence it fails.

So you should install blinker first. However, I think that Flask-Security code should also check for blinker before trying to use connect_via.