Since everyone had voted for Byakuyako, I'd add my own list:
Zeni Geba
Kono yo no Hate
Koukou Kyoushi (the 1993 version)
Iguana no Musume
Suzuki Sensei (not that dark, but still somewhat gloomy)
Atami no Sousakan (though it tends to drift towards satire-surreal, black comedy)
Lost Days (again, not too gloomy, it's much intense rather than dark)
Marks no Yama (if you're fond of thriller, fast-paced drama)
Shokuzai