Historical Rules · 25 August 2017
Rule 509.1b
Official source text from the 25 August 2017 Rules release
The defending player checks each creature he or she controls to see whether it’s affected by any restrictions (effects that say a creature can’t block, or that it can’t block unless some condition is met). If any restrictions are being disobeyed, the declaration of blockers is illegal. A restriction may be created by an evasion ability (a static ability an attacking creature has that restricts what can block it). If an attacking creature gains or loses an evasion ability after a legal block has been declared, it doesn’t affect that block. Different evasion abilities are cumulative. Example: An attacking creature with flying and shadow can’t be blocked by a creature with flying but without shadow.