If I have an Eternatus VMAX with "Eternal Zone" Ability and less than 8 Pokemon (all Darkness type) on my bench, can I use Giratina's "Distortion Door" Ability to put it from the discard pile onto my bench and use its effect before it is discarded?
No, you cannot even put it onto your bench because it is a non-Darkness Pokemon.
Source: Darkness Ablaze FAQ (2020-07-31), TPCi Rules Team (2020-08-13)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH3_117
SWSH3_117
SWSH3_192
SWSH3_192