Dark Star has a no albedo whatsoever, since it is purely black on the outside, hence its name. The planet's size is unknown, but judging by how it covered a huge chunk of Ripple Star's atmosphere as Ribbon watched the planet's creation, Dark Star can be assumed to be very large, about Jupiter's size in order for the planet's disk to be visible at that distance.
Not much is known about the planet's atmosphere, which obscures the surface from view, but Dark Star's insides are a roiling chaos of reds, blacks, and purples. The planet's gravity seems to match Pop Star's.
Dark Star's surface has technically never been seen.