If I'm wrong please correct me, but have you written these sentences by your own?
Because if I'm not entirely mistaken, using the past tense at the end does not sound entirely correct (or at least natural).
My advice would be, either to use the same grammar as you:
a)夜になると、とても暗い・暗くなる
b)夜になって、とても暗くなった
Or change the grammar in the first part
a)夜になってからとても暗かった
b)夜になったらとても暗かった
Where から would translate to "since" in English, and たら形 marks past tense.
The problem in the second sentence is that you need an indicator of change on both sides of the て.
It would then translate to something along:
"It became very dark when evening fell." (literally "when it became evening" but that doesn't sound right. You'll often have that problem with translating Japanese.)
This is a bit more than you were asking for, but I hope it was of some use to you! If you don't understand, just let me know