How To Truncate Using Site Builder Item List
Using Jamroom
Then you would do that in your item_grid.tpl file. make a copy of that in your skin and remove the tpl_dir="jrBlog" so that it does use the one in the skin.
From there, the code your after is the same code thats in the lists, so check out the jrBlog item_list.tpl file to see how it is making the blog post get cut off. It will be a |something on the blog output variable.
so something like:
{$item.blog_text|jrBlog_readmore}