Don't assume I know what I'm talking about, because I'm new to RoR, but I just spent the last few hours reading up on the asset pipeline after running into problems with it. I'll share a few things I've learned that might help you conceptualize:
Anything in public/ is left just the way it is, and server as static files directly by the web server. There are two points worth considering regarding public/ assets, though:
1) They don't get the benefits of precompiling, which include:
1a) fingerprinting - Appending an md5 hash based on file contents to the filename, so that the filename changes when the file changes, forcing caches to reload. This is useful if the file might change some day (a new version of blueprint, in your case).
1b) concatenation - The precompiler can/will combine multiple CSS or JS files into one, which makes the download faster. (Exactly what files get compiled and into how many is configurable.)
1c) minification - The precompiler removes whitespace (and other clever optimizations) to shrink down the size of your CSS/JS files.
2) I'm still trying to figure this part out, but whether something is in /app/assets and goes through precompile affects whether and how helper methods work (things like asset_tag, image_tag, and javascript_include tag, which you use in your views).
Even though I'm totally unqualified, I'm seriously considering starting my own Rails Assets Best Practices page on a wiki somewhere to start organize my thoughts. I think it's sorely lacking - I've had to dredge bits of knowledge from many places, and some of what people are suggesting I find objectionable (like modifying config files to precompile add unmanifested assets).
