An example would be Erlang which is a runtime built from the ground up to support lightweight threads where reading or writing to a socket of using the built in APIs won't block the underlying OS thread. You can still sabotage the runtime by writing your own native library and doing blocking operations from there, but the pitfalls are more obvious in that scenario.
If the underlying concurrency and network/disk IO primitives are truly non-blocking for lightweight threads then everything built on top of them will also be non-blocking. Non-blocking for the underlying OS thread that is.
That is where co-routines and other co-routine like libraries are a leaky abstraction. If you step into one of those blocking APIs in a third party or standard library you will block the underlying OS thread. This means you can't integrate with existing code or tools easily and you are always vulnerable to accidentally doing so.