Are you including the ie.css file? Below the regular blueprint css files, include the ie.css
file, using the notation that only IE will parse:
<link rel="stylesheet" href="../../blueprint/screen.css" type="text/css" media="screen, projection">
<link rel="stylesheet" href="../../blueprint/print.css" type="text/css" media="print">
<!--[if lt IE 8]><link rel="stylesheet" href="../../blueprint/ie.css" type="text/css" media="screen, projection"><![endif]-->
Make sure that all the URLs to the CSS files are correct and are working on your site also.