Approach 1

Override all widgets of a given type in a form. In this example we set CKEditor widget for all textfields in the form.

admin.py 
class BlogAdmin(admin.ModelAdmin):

    formfield_overrides = {
        models.TextField: {"widget": CKEditorWidget},
    }

Approach 2

Override widget only for a desired field in the form.

forms.py 
class BlogAdminForm(ModelForm):
    class Meta:
        model = Blog
        widgets = {
            "body": CKEditorWidget(),
        }

admin.py 
class BlogAdmin(admin.ModelAdmin):
    form = BlogAdminForm