An attacker is classified as "blocked" or "unblocked" based on decisions made during "Declare Blockers step" while the ability already triggers during "Declare Attackers step".

The only consistent way to interpret this card text is to say a creature is "unblocked" unless otherwise blocked making the word "unblocked" superfluous since they couldn't have been blocked by the time this triggers.

Alternatively: "Whenever a creature an opponent controls attacks and isn't blocked, that player discards a card."

Or a Ghostly Prison varition? "Each creature attacking you is blocked unless its owner discards a card" ?

Thanks, I'm not very good at rules text.