That sounds like a very moving story. But I don't have enough info to rate the effectiveness of your proposed title, nor to suggest another.
For me, the most engaging and satisfying novel titles hint at, refer to, or are explained by something revealed in the second half of the book, and are unique to the world (for non-real-world stories), characters, or plot. A promise or question of sorts. Even better are those titles which somehow (usually through symbology) add a level of angst or depth of understanding once the relationship to the characters or plot is fully understood. To Kill a Mockingbird, People Who Eat Darkness, The Unbearable Lightness of Being, etc. resonate deeply with me, for various reasons, and intrigue me enough to learn more. Less interesting to me are the overt, such as The Girl With the Dragon Tattoo, A Tale of Two Cities, and Seven Years in Tibet. They serve, but they neither inform nor excite.
I sense your story will address the protagonist's conflict of love and desire with guilt, obligation, anger, and grief (the "black"?). Depending on her choices and outlook, "light" may be the perfect counterpoint. But in this case I'd feel the proposed title would be weak. A story that promises so much depth and emotional turbulence should be served by a title as interesting. Maybe something in the spirit of "Black and Light", but "deeper".
But if the "Black and Light" refer to some specific event, object, location, etc. in the story, and this "thing" is deeply symbolic or meaningful to the character and her choices, then I'd say you've nailed it.