Noob Question: Is the intention of the cSS API to make it possible to use the components as normal HTML without the need for any additional React code? Does the usage of only the CSS API therefore require the inclusion of the React JS package or can the Blueprint JS package alone work ?
As an author of Blueprint, I can confirm that it is in fact possible to use some of the Blueprint components via CSS only. Components that describe a CSS API in the documentation can be used without React, but usually with caveats (the biggest being that you must write the markup correctly). The CSS API is presented as an alternative to the JavaScript (React) API, and CSS modifiers are supported by the JS APIs through the className prop.
As Blueprint is a React-based UI toolkit, the best experience will be had by using React, primarily because you no longer have to write the markup correctly.