I may understand issue #462 all wrong, but I thought scenarios like this simple example should be covered by it. (Running this example in freshly created boot2docker v1.10.3 VM)

1/ Pull registry and another arbitrary image:
docker pull distribution/registry:master
docker pull ubuntu

2/ Run registry:
docker volume create --name registry
docker run -d -p 5000:5000 -p 5001:5001 -v /root/registry-config.yml:/etc/docker/registry/config.yml -v registry:/var/lib/registry --name registry distribution/registry:master

3/ Tag both those images as "test:lates" and push them to registry to ensure there will be some unreferenced blobs:
docker tag distribution/registry:master localhost:5000/test
docker push localhost:5000/test
docker tag ubuntu localhost:5000/test
docker push localhost:5000/test

4/ Now there should be unreferenced blobs in my registry right? Stop registry. Run garbage collection:
docker stop registry
docker run -v /root/registry-config.yml:/etc/docker/registry/config.yml -v registry:/var/lib/registry --rm distribution/registry:master garbage-collect /etc/docker/registry/config.yml

The blobs were not deleted. Am I missing something here?

Thanks!