When a file is uploaded to Nexus Repo it is first written into a temporary file in the blob store's "tmp" directory. Once it is completely written and closed it is then renamed to its final destination. This ensures there can never be any partially written content in blob storage.
In some rare cases these temporary files can get stuck. For instance, if Nexus Repo crashes.
It would be a good idea to clean out blob store "tmp" directories at startup to reclaim space that is taken up by those 'lost' files.