Android : Generate blob upload URL from Blob Key googleblobstore

I am trying to upload blob to google blob store I am using this method to get upload Key

BlobKey key = blobstoreService.createGsBlobKey("/gs/saad");

I wonder how to create upload URL from this key ?

I know that i can get URL directly using the line bellow

String blobUploadUrl = blobstoreService.createUploadUrl("/saad");

but in this case I can't get the blob key which I need to store in my database in order to be able to download the file later.