how come if ætherflame wall gains shadow it loses the ability to block creatures with shadow?
When something happens in Magic “as though” something were true, then that thing is treated as true for those purposes.
Creatures without shadow can’t block creatures with shadow. Similarly, creatures with shadow can’t be blocked by creatures without shadow.
Aetherflame Wall can block creatures with shadow as though they didn’t have shadow. Normally this allows Aetherflame wall to block those creatures because a creature without shadow (Aetherflame Wall) is blocking a creature without shadow (The creature with shadow being treated as though it didn’t have shadow.
However, if Aetherflame Wall has shadow, then it can’t block any creatures. It can’t block creatures without shadow because it has shadow. It can’t block creatures with shadow because its ability causes it to block them as though they didn’t have shadow, and that makes the block illegal.
