Sounds/looks like a problem with the theme... To test this, try changing themes (Administer > Site building > Themes) to something like Garland. Test the blog site with that theme, and if it works alright, it's a problem with your Tapestry theme.
I've had a look at the code in page.tpl.php for the Tapestry theme on the site you linked to, but they've either not used a standard coding practice, or I'm just not familiar with their coding style (i.e. I couldn't make head or tail of it).
One thing to check for would be the existence of a .tpl.php file for the blog page (possibly from a view). It doesn't seem to be the same as the normal page.tpl.php file, hence the weird look.
Sorry I couldn't be of more help...
I am using the tapestry theme from www.roopletheme.com at http://gbacc.net. When you open a particular blog page it appears just fine, but when you go to http://gbacc.net/blog/ the page appearance is all messed up and I am not sure how to fix it. Any Ideas?
~Phil