Here is how:

Frieren is not a pure elf, she is actually half-elf and half-demon.
Her mom was the only demon who never ate humans and was actually a good person.

Imagine in the huge battle where Frieren can't win no matter what, and then her demon powers unlock and then she can use forbidden magic.
Then when Frieren transforms, her hair becomes blacks, horns grow on her head, fangs in her mouth. Also she gains a devil tail and also wings.