In your code behind
protected string PostUrl = string.Empty;
in PopulateControls set this variable:
PostUrl = SiteRoot + blog.ItemUrl.Replace("~/", "/");
then you can use <%= PostUrl %>
Hope it helps,
ps the developer forum would have been a better place to ask this question
Note to other readers, I do not recommend modifying the mojoPortal source code, Nitin has cloned the blog which is the only recommended way for customizing a feature while keeping it possible to upgrade to new versions of mojoPortal.