If you have 2 Darkness Pokemon on the bench and an Eternatus VMAX with the "Eternal Zone" Ability, can you play a non-Darkness Pokemon onto your bench?
No, you cannot put any non-Darkness type Pokemon into play. This includes things that would put Darkness type Pokemon directly into play, not just playing them from your hand!
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