It was 2 AM and all dark.
Is all dark grammatical and interchangeable with completely dark in this context?
No. The sentence is not colloquial. I'd say 'pitch black'.
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These days 'all dark' is much more closely associated with biologists' descriptions of the color features of various species:
... Tibiae without distinct basal pale bands; wing scales all dark ...
from The Culicidae of the Australasian Region
CJ