I am specifically referring to InputStream (Java SE) and its implementations.
How is blocking performed? I'm a little worried that they use a "busy-waiting" mechanism, as it would produce a lot of overhead. I believe they do it another way, but I'm just asking to be certain.
No, they don't use busy-waiting. It's up to the implementation (which will depend on the kind of input stream you're using), but generally it's likely to use a blocking OS-level API, so the efficiency will depend on the OS.