Shadow is an evasion ability defined in CR 702.28. It imposes two restrictions: (1) a creature with shadow cannot block creatures without shadow, and (2) a creature without shadow cannot block creatures with shadow. Both sides of the ability work symmetrically to keep shadow creatures in their own 'plane.'
During the declare blockers step (CR 509.1), a player assigns blockers. A block is only legal if all blocking restrictions are met. Because a creature with shadow is explicitly restricted from blocking creatures without shadow (CR 702.28b), assigning such a block is illegal and the game simply does not allow it.
It is worth noting that if a creature somehow has both shadow and another ability that grants it the power to block as though it didn't have shadow (or similar override text), that specific card's text would take precedence — but no standard shadow creature has such an override.
Concrete example: Your opponent attacks with a 2/2 Grizzly Bears (no shadow). You control a Dauthi Slayer (shadow). You cannot declare Dauthi Slayer as a blocker for Grizzly Bears; the block would be illegal under CR 702.28b, and Grizzly Bears deals its combat damage unblocked.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.