What do you want to find? 

You could use IContentModelUsage like below;

var contentTypeRepository = ServiceLocator.Current.GetInstance<IContentTypeRepository>();
var repository = ServiceLocator.Current.GetInstance<IContentRepository>();
var contentModelUsage = ServiceLocator.Current.GetInstance<IContentModelUsage>();
var myblockType = contentTypeRepository.Load<MyBlockType>();

List<ContentReference> myblockTypeReferences = contentModelUsage.ListContentOfContentType(myblockType).Select(x => x.ContentLink.ToReferenceWithoutVersion()).ToList();

then you could iterate through your references to get the data you need. 

foreach(ContentReference cref in myblockTypeReferences) {

MyBlockTypemyblock;
repository.TryGet<MyBlockType>(cref, out myblock);
}

or maybe IContentRepository.GetDescendents(ContentReference.BlockFolderToLookIn)

I'm don't think there is a way to search for blocks the same way as FindPagesWithCriteria