You gave very general description of the situation. here is at least several factors that can influence her attitude toward ‘Dark Arts’
When you say she is spiritual but not religious, what that mean? She grew up in non religious home? She was religious before but gave it up at some point? Is she resentment toward religion at all, or any specific religion? Find why she finds your Dark Arts childish?
Another thing is how you present yourself and present dark arts to her? Have you described what you do in competent way? or vague? Have you shown here parallels between your practice and her spirituality? Have you tried to descript ‘Dark Arts’ in terms that she can rely to? (like i.e. Crowley whose parallels between Yoga ang Magick helped me to describe magick to New Agers in a way that didn’t scare them)
Third thing is what are her associations toward ‘Dark Arts’ even if she is not religious and is spiritual, she can have some fears. Maybe she feel insecure about herself in contact with dark arts, maybe she feel insecure of you when you practice dark arts? Check on her intention when she mocks you, is she caring about your good? Or maybe she feels she have to compete with Dark Arts about your attention? etc… As you can see there is many options. Honest talks will be good, assurance from your side too.
And BTW there is still one other thing, maybe she is mocking you in a friendly way while you are touchy over Dark Arts topic?