Do you consider reading from a normal array (one not backed by a memory mapped file) to also be blocking?
If you consider all memory reads to be "blocking", then everything must be "blocking". The executable code must, after all, be read by the processor. In an extreme case, the entire executable could be paged out to disk! This interpretation is not what most people mean by "blocking."
I never rely on synchronous IO being non-blocking when writing regular code (ie not embedded). As such reading from cache (non-blocking) vs disk (blocking) doesn't matter that much, as such. It's synchronous and that's all I need to reason about how it behaves.
If I need it to be non-blocking, ie playing audio from a file, then I need to ensure it via other means (pre-loading buffer in a background thread, etc etc).
edit: And if I really need it not to block, the buffer needs to reside in the non-paged pool. Otherwise it can get swapped to disk.
This hints at a way to make it work, but would need the compiler (or explicit syntax) to make it clear you want to be able to switch to another task when the page fault triggers the disk read, and return to a blocking access that resolves the read from memory after the IO part is concluded.
It could look like a memory read but would include a preparation step.