It's not that her manga are particularly "dark" (her darkest is World Without Freedom and I've read much darker yuri) or "realistic" (Kodama's manga are largely soap operas), but that they invite the reader to root for and be invested in manipulative women who deliberately use their beauty, feminine charms, and interpersonal skills to their own advantage, and that is a personality type most people have been socialized to despise on sight.
In fact, in the afterword to “I Married My Best Friend” Kodama draws herself as two avatars, “light Kodama” and “dark Kodama,” (the familiar “good angel/bad angel” images), with “Gentle Romance,” “Silly Humor” and “Boobs” listed for the light version and “Soap Opera,” “Jealousy,” and “Heavy” for the dark, with a footnote saying “Dark Kodama draws boobs, too.”
I’m not sure that completely tracks with Kodama’s body of work (except for the boobs), but that’s how they present it, anyway.