code: func (c *Corpus) EnumerateSingleBlob(fn func(camtypes.BlobMeta) bool, br blob.Ref) {
  	if bm := c.blobs[br]; bm != nil {
  		fn(*bm)
  	}
  }
docs: // EnumerateSingleBlob calls fn with br's BlobMeta if br exists in the corpus.