You can use FindPagesWithCriteria

 IPageCriteriaQueryService queryService = ServiceLocator.Current.GetInstance<IPageCriteriaQueryService>();

var contentLoader = ServiceLocator.Current.GetInstance<IContentLoader>();
contentLoader.GetDescendents(new ContentReference(blockFolderId))
  .Select(blockReference => contentLoader.Get<IContent>(blockReference))
  .Where(block => block.Name=="MyBlock")

Something like that...

Probably want to add a content reference instead to it somewhere if possible because of performance of above code...

IPageCriteriaQueryService doesn't work for blocks. Do you Episerver Find? Otherwise you have to do as Daniel suggested.