If you set null=True, it will allow the value of your database column to be set as NULL. If you only set blank=True, django will set the default new value for the column equal to "".
There's one point where null=True would be necessary even on a CharField or TextField and that is when the database has the unique flag set for the column. In this case you'll need to use this:
a_unique_string = models.CharField(blank=True, null=True, unique=True)
Preferrably skip the null=True for non-unique CharField or TextField. Otherwise some fields will be set as NULL while others as "" , and you'll have to check the field value for NULL everytime.
