In the main area of night less city there is a exit to the north you have not been through (you can see it on the map). You already have the item you need to get through the door.
Viewing post in The Legacy of Hedonia: Forbidden Paradise (alpha demo) comments
My first guess would be secundas study (she might be in the vdsm room), second have you been to stratum 3 it might be a requirement, second have you done stratum 4 sequence 1 the magician (could be a requirement due to reasons), is blanche still in the cafe? You may have to score high enough on the mini game for her to be let go (speculation), failing that check all maps for blue and red exclamation marks. If that fails I don't know you may have found a bug.