ByArgo.cs

private static void Before_Writing(ISitemapGeneratorBeforeWriting evt)
{
  // gets the entries that are about to be written in the sitemap
  var entries = evt.Entries.ToList();

  // gets the full Url
  string hostUrl = SystemManager.CurrentHttpContext.Request.Url.ToString();

  // get all the blog posts
  BlogsManager manager = BlogsManager.GetManager();
  IQueryable<BlogPost> posts = manager.GetBlogPosts()
            .Where(p => p.Status == ContentLifecycleStatus.Live && p.Visible == true);

    // add blog posts pages to sitemap
    if (blogPosts != null)
    {
      foreach (var bp in blogPosts)
      {
        // build the Url
        string blogPostUrl = string.Format("{0}/{1}",hostUrl + BlogDetailUrl, bp.UrlName);

        if (!string.IsNullOrEmpty(blogPostUrl))
        {
          // Create new sitemap entry for each custom url
          SitemapEntry newSitemapEntry = new SitemapEntry();

          // Set the entry location – (the <loc></loc> tag) - required
          newSitemapEntry.Location = blogPostUrl;

          // Set the other XML tags - optional
          newSitemapEntry.Priority = 0.5F;

          // Add this new entry to the list of entries to be written by//sitemap generator
          entries.Add(newSitemapEntry);
        }
      }
    }

  evt.Entries = entries;
  }

private const string BlogDetailUrl = "blog/post";