Yep, both the CSS and JS is included on every blog page. (The only caveat with the JS is that your Preferred URL in your blog settings has to be yourblog.writeas.com or a custom domain.)
But if you want different code to run on different pages, you’ll check something like the window.location.pathname for what page you’re on, e.g.
if (window.location.pathname == '/') {
// Blog index page
} else if (window.location.pathname.lastIndexOf('/tag:') == 0) {
// Blog tag pages
} else {
// Blog posts
}