Hi,
You are right, it was preventing posting the comment on postback, but it should not postback. This is now fixed in the source code repository and on the demo site. You can easily fix it in your copy by editing /Blog/Controls/BlogViewControl.ascx
Find this:
<portal:mojoButton id="btnPostComment" Runat="server" Text="Submit" />
and change it like this:
<portal:mojoButton id="btnPostComment" Runat="server" Text="Submit" ValidationGroup="blogcomments" />
Best,
Joe