Yes you most certainly want to and need to water the poinsettias even when they are in the dark.
Having the poinsettias in the dark and watering them in the dark is very important and vital to allow the poinsettia to bloom.
If you just sit the poinsettia plant in the dark and do not water when needed then the poinsettia will likely die and not bloom at all.
Just water the poinsettia enough to allow the soil to become wet and then when the soil of the poinsettia becomes dry then you water it again.
Just don't over water the poinsettia otherwise you risk the poinsettia dying due to root rot.