Since you're asking for constructive criticism I'll try as a viewer to give you my opinion on what you might work on.
First of all... avoid making simple mistakes such as the scene when the black blob spits out those red feathers. The protagonist at first draws out his LEFT arm and then, in the next scene... he catches the feathers with his RIGHT hand.
Second thing. The black blob was done pretty well. It had a good 'fluid' motion to it and all, but the protagonist, on the other hand, seemed a bit too stiff.
Third and last thing. The cape of the protagonist MUST be done better since it's practically the character's main garnment. Being a serious character you should also work on improving the expression (and 'depth') of his eyes. Needless to say that by increasing the quality and detail of your characters you'll increase the viewer's interest towards them.
Story-wise, I have a feeling you won't have problems. Due to the abstract nature of this flash things worked out fine, but make sure to add a preface for future ones.
Keep up the good work.
(btw, have you been influenced by Naruto anime in your flash?)