Hello,
You said you sent 120 flowers, but where did you send them? I don't understand. Did you choose the “good” or “bad” path? Did the “hero” tell you to take those flowers? There are often clues that tell you when to take the flowers. And which event are you on?
As for the Black scene, it’s because you loaded an old save file into a newer version of the game. Since some images have been changed and new events have been added, the game can’t find the old images because the save file is incompatible.
I recommend creating a new save file to avoid the black screen and checking the walkthrough on Patreon to find out what to do with Mula.