There was a packaging error in the 2.3.8.1 release where that file was left out of the package, if you upgraded it should already be there but if you did a clean install of 2.3.8.1 it is missing.
You can solve it by creating a text file in the Blog folder named BlogSiteMap.ashx, and put this as the content of the text file using a text edito like notepad:
<%@ WebHandler Language="C#" CodeBehind="BlogSiteMap.ashx.cs" Class="mojoPortal.Web.BlogSiteMap" %>
Hope that helps,
Joe