Google Cloud Storage seems more cost effective than the App Engine Blobstore.
At the moment I am storing user-uploaded image files as Blob
I try to give a different answer. I would suggest Picasa as the image storage in common situation.
I ran into the same question when Google stopped to serve Google Storage for free. But then I noticed that Picasa is free for pictures smaller than 2048x2048 when your Google plus account is enabled. And it's still more cost-effective for paid-storage.
So if you are going to store small pictures like profile pictures, I would suggest App Engine Blobstore. If you are dealing with some more photos and pictures, take a look at Picasa.