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When determining if a monster’s effect should be negated by “Shadow-Imprisoning Mirror”, check the monster’s Attribute at the time its effect is resolved.
This doesn't stop monster effects from activating. It just negates any effect which does activate (on the field/GY).
When a Non-DARK monster’s effect is activated while the monster and “Shadow-Imprisoning Mirror” are both face-up on the field, if you chain “DNA Transplant” and select “DARK”, the monster’s effect will be negated if the monster is still face-up on the field when its effect resolves because it is now a DARK monster.
Even if the Physical card is no longer on the field/GY, the effect is still negated.
“Shadow-Imprisoning Mirror” cannot negate the Continuous Effects of DARK monsters.