You are right, you don't want to make the changes to the thematic parent theme. Make a copy of the template-page-blog.php and place that copy in your child theme. Now the copy in your child theme will overwrite the parent copy. That is the safe way to make changes to your php files.
Look for this code at the bottom of the template-page-blog.php that you copied into your child theme.
'<?php
// action hook for placing content below #container
thematic_belowcontainer();
// calling the standard sidebar
thematic_sidebar();
// calling footer.php
get_footer();
?>'
Just comment out the side bar so the code looks like this
// calling the standard sidebar
// thematic_sidebar();
That will remove the side bar but you will probably need to make some other CSS changes to achieve the desired aesthetic.