Hi,
to display different message on 0 you can use ifCompare function:
<%# IfCompare(1, 2, "The values are different", "The values are equal") %>
It would look like this:
<%# IfCompare(BlogFunctions.GetBlogCommentsCount(Eval("DocumentID"), Eval("NodeAliasPath")), "0", BlogFunctions.GetBlogCommentsCount(Eval("DocumentID"), Eval("NodeAliasPath")) + " comments", "Add comments") %>
Replace string "Add comments" with your code for reaching comment section.
Hope this helps.