The term "blocking" in UNIX-like OSes is jargon with a particular meaning. It means an interruptible wait.

Disk files do not block - they may Disk Wait instead, which is an uninterruptible wait (this is what the 'D' process status stands for). Disk Wait doesn't interact with O_NONBLOCK, select(3), poll(3) etc.

(Back in the bad old days it wasn't even possible for a Disk Waiting process to wake up to process a SIGKILL and die, which was the bane of system administrators everywhere when NFS introduced the idea of disks that could disappear when the network went down. Now it's common for OSes to make some kinds of Disk Waits at least killable).