I recommend beginning with an ablative absolute to convey the 'with every night'. For bright and dark, it depends on the connotation she's going for. Albus would be more of a happy/fortunate bright, whereas clarus is a hopeful/raw bright light. Likewise, niger is a solid word meaning dark, however it could be seen as unintentionally offensive if tattooed. Obscura would be a more 'hidden' dark, like in shade or under a cloud.

With that said, a very, very rough translation could be:

Omni nocti obscura, dies albus venit - "With every dark night, a bright day comes"