It's meant to be grainy during night scenes, because they are shot on digital video (a similar look to what Michael Mann used on Cillateral a few years back).
You should not assume that the poing of blu ray and hi def is to make everything look pin sharp and crystal clear...if the film was made to look intentionally grainy, then that look will be preserved on the blu ray release.
A few recent examples are Saving Private Ryan, Minority Report and to a certain extent War Of The WorlRAB...all Spielberg movies that utilised that somewhat grainy, slightly washed-out look.
By all means a good transfer should remove grain, dirt and scratches etc if they are simply the result of wear and tear over the years, but it should also preserve the filmakers original intentions.