lambda *args, **kwargs: None

According to PEP8, you should "Always use a def statement instead of an assignment statement that binds a lambda expression directly to a name." So, one thing I would change is:

def blank_fn(*args, **kwargs):
    pass

However, I think a more pythonic way to do this is:

def perform_task(callback=None):
    print 'doing stuff'
    if callback is not None:
        callback('x', 'y', z='z')

There shouldn't be any need to call a function that does nothing. Truth value testing is cheaper than function calling.

def do_nothing(*args, **kwargs): pass
def do_something(arg, callback=do_nothing):
    a = 1 + 2
    callback('z', z='z')
def do_something_else(arg, callback=None):
    a = 1 + 2
    if callback is not None:
        callback('z', z='z')

%timeit do_something(3)
1000000 loops, best of 3: 644 ns per loop

%timeit do_something_else(3)
1000000 loops, best of 3: 292 ns per loop