You are right, but I supposed that min_length and allow_blank won't be used together.

May be such checking in __init__ is what you mean:

if self.allow_blank and self.min_length:
    try:
        assert 0 == x.min_length
    except AssertionError:
        raise Exception("'allow_blank'=True conflicts with 'min_length' = {0}. If 'allow_blank' is True then 'min_length' should be equal 0.".format(self.min_length))