Although the creature that enters the battlefield is blocking, it was never declared as a blocking creature for the purposes of abilities that trigger whenever a creature blocks (such as Aetherplasm's ability, for example).
Aetherplasm
If you want, you can return Aetherplasm to your hand and not put anything onto the battlefield. The attacking creature will still be considered blocked, so it won't be able to deal combat damage to you unless it has trample or a similar ability.
Aetherplasm will return to your hand before any combat damage is assigned.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Nottsuo
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