Yes, you can access blobs too. I've been using the HTTP API (the one that backs CloudKitJS) to process data in app engine. So far I just used the JS API token for authentication. Records with an associated blob asset provide a temporary accessible URL to the asset which is usually stored on S3.
Not sure about latency of the requests, never had a problem with that. More often the problem was that I had confirmation my client software had written data to CloudKit but it might take a while before the server can actually query for it. (Could be an indexing delay haven't checked if the object is immediately available using the object ID)