Google Cloud Storage seems more cost effective than the App Engine Blobstore.
At the moment I am storing user-uploaded image files as Blob
The Blobstore is tightly integrated with App Engine, while Cloud Storage is offered stand-alone. Otherwise, they look like different interfaces to the same basic service.
I don't see any good technical reason for them to be priced differently; perhaps it's a marketing thing.
The Blobstore has a more mature API and does some neat stuff like making thumbnails on the fly without incurring extra storage. The Cloud Storage API is experimental and rapidly changing. They're both perfectly capable of storing large images. To choose, you'll need to decide if the Blobstore's more rubust API justifies the nominally higher cost.