Nit: `.blank?` is a test for `empty? || nil?`  In this case you only need the `nil?` part.  So `.blank?` is overkill.
But I think it's harder than this because you haven't handled the case where you save the object, have you?  Try a test for that.  I think you'll find it fail because it will find itself on the save!

Rails `validates_uniqueness_of` handles that case by excluding `this` from the uniqueness test.  Why can't you use that?  Using the Rails 3 notation:
```ruby
validates :section, :uniqueness => {:scope => :event}
```