I’ve added the conditional CSS include tags for IE into the head of my document:
<!--[if lt IE8]>
<%= stylesheet_link_tag 'blueprint/ie' %>
<![endif]-->
Unfortunately this causes them to show up in the page when it’s rendered in IE.
How can I banish them?

I think the right syntax is
Note the space between ‘IE’ and ‘8’. Not sure if this is your problem – but try it. Check this page for specifics of the conditionals.