I am attempting to Get a list of blocks that not being used.   

 [ServiceConfiguration(typeof(IContentQuery)), ServiceConfiguration(typeof(IContentSlice))]
    public class MyNewSlice : ContentSliceBase<SitePageData>
    {
        private readonly IContentRepository _contentRepository;

        public MyNewSlice(IClient searchClient, IContentTypeRepository contentTypeRepository, IContentLoader contentLoader, IContentRepository contentRepository) : base(searchClient, contentTypeRepository, contentLoader)
        {
            _contentRepository = contentRepository;
        }

        public override string Name => "My New Slice";

        protected override ITypeSearch<SitePageData> Filter(ITypeSearch<SitePageData> searchRequest, ContentQueryParameters parameters)
        {
            return searchRequest.Filter(x => x.MatchTypeHierarchy(typeof(BlockData)) & _contentRepository.GetReferencesToContent(x.ContentLink, false).Any().Match(false));
        }

        public override int SortOrder => 505;
    }