Yes, metaphorical. Probably best way to equate it is the stars represent some level of inherent charisma and personal magnetism that Ai possessed. A "star power" or "stage presence" and an intense desire to be admired/loved, preferably on stage. Ai, Ruby, and Aqua are always going to be people that your attention will drift towards.
When it goes black, it's more like an inner fire. Not always murderous hate, but a will to crush someone. Using that natural charisma for negative means.
Ruby and Aqua are about half as inherently charismatic as Ai was (hence each one gets one), but still much moreso than most others.
Similar effect is Kana's galaxy within her pupil.
No supernatural powers behind it, just artistic license.