Niji no Majo to Enogu no Ouji-sama

The protagonist is a witch who lives in a painted land.
But she is very bad at using magic, and whenever she fails, she causes rain to fall.

One day, the protagonist was practicing magic, but once again, the magic fails and rain starts to fall. While drearily taking shelter from the rain in the forest, she met a boy with black hair and black eyes.

[translated from Freem!]