I don't think that's possible, but I could be wrong. Most x-cart sites that I've seen with blogs have 2 separate log ins.
As far as a 3rd party mod doing this I personally have not seen one like what you are asking for.
Most people who add blogs to their x-cart site host it at
www.yourdomain.com/blog and edit the blog's CSS to match the x-cart site.
If you do find a mod like what you are asking for please be sure to post your find as I would like to check it out.