hi ans,
is it sure from context, that there is not meant a special kind of darkness: an eclipse ? ( = die Finsternis ) - this is a short time of darkness.
"über" has the meaning of 'during' - so the English equivalent also could be: but during the time of the eclipse, one does...
When speaking of a long time of darkness - as during antarctic winter months - you'll skip off the article and write: "aber in der Zeit während Finsternis ist, sieht man nichts"...
The quoted sentence is not 'pure' German at all - it's a kind of spoken German.

behe (cw)