I am trying to write an Android application that will take a picture, put the data (byte[]) in an object along with some metadata, and post that to an AppEngine server where
The Android Asynchronous Http library provides a method of uploading files to a web service. I've used it in my projects with great success.
Sounds like you need a web service method that takes an ID parameter and returns a file. You would store your blobs in the database table indexed with ID so that they can be referenced.