Hi,
You can't change the content store for an existing implementation; all the content that was stored on the filesystem needs to be available to Alfresco, still. Either start with a new system, or use the org.alfresco.repo.content.replication.ReplicatingContentStore with your blob store as the primary store, the old file content store as a secondary store, and only replicate inbound. If you want to ditch your file store but need to pull in all the content into the blob store, use the org.alfresco.repo.content.replication.ContentStoreReplicator by activating the quartz job org.alfresco.repo.content.replication.ContentStoreReplicator$ContentStoreReplicatorJob.
Regards