I am trying to load a CSS framework, Blueprint, onto my Rails 3.1 application.
In Rails 3.0+, I would have something like this in my views/layouts/application.html.e
The end result of a translated SASS files is actually css file, so it shouldn't change how you include your stylesheets.
Additionally, just because the SASS gem is enabled doesn't mean you can't use plain vanilla css files simultaneously. Therefore you should have no problem including the blueprint css files.
However, if you want to go purely SASS, I recommend checking out the compass gem which has nice support for blueprint:
http://compass-style.org/
It also contains support for ie-specific stylesheets and macros.