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This is not a bug, of course, but is fully explained in the documentation. From the field options docs:
If a field has blank=True, form validation will allow entry of an empty value. If a field has blank=False, the field will be required.
The key is the reference to validation. The docs also, naturally, have a whole selection on data validation. Again, from that page:
Note that validators will not be run automatically when you save a model, but if you are using a ModelForm, it will run your validators on any fields that are included in your form.
So, as the model validation page explains, to apply the validation for blank=False you should run the model’s full_clean() method.
(In any case, there’s no point in complaining about a possible bug in a version that’s two releases old: if you think there is a bug, you should upgrade to the latest version and see if it has been fixed. Anyway, it’s not a bug.)