`Blob` uses the description from `BlobModel` in its JSONSchema

[rwb27]

Currently, the description for Blob objects in JSONSchema/ThingDescription is:

A model for JSON-serialised .Blob objects.

This model describes the JSON representation of a .Blob and does not offer any useful functionality.

This is the docstring for BlobModel and it's a reasonable docstring, but it should be replaced with something more helpful for clients in the schema.