Django（1.9）makemigrations在布尔字段上没有得到Blank = True

I have the following model:

我有以下型号：

class Site(models.Model):
    """
    Model for a site entry
    @author: Leonardo Pessoa
    @since: 05/09/2016 
    """
    from decimal import Decimal

    consolidated_financials     = models.BooleanField(blank=True)
    type                        = models.ForeignKey(Type)
    tier1_business              = models.ForeignKey(Business, limit_choices_to = {'tier': 1}, related_name='%(class)s_tier1')
class S