Here is the full list of all 9 Blooming Nightshade achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Got through the first month of the semester.
You couldn't find or keep a romance alive until the end of the year...
Spend Christmas Eve with Izumi.
Spend Christmas Eve with Kiyomi.
Spend Christmas Eve with Nanami.
Spend Christmas Eve with Atsuko.
Spend Christmas Eve with Kiku.
Spend Christmas Eve with Shiori.
Witnessed two of your friends become a couple.