I suspect black plants would more likely evolve on worlds with less light available (orbiting brown dwarfs, or in really distant orbits, for example), than on worlds with plenty of light. the fact that plants have a specific color on earth ...conclusively proves that plants get more than enough sunlight than is needed to survive and spread, so much more that they can reject a significant fraction of that sunlight by reflecting it. unless the biochem driving extraterrestrial photosynthesis is much less efficient than that our own world's plants, this means that a planet with markedly more light available than earth will also be able to reject wavelengths by reflecting a color, and easily not miss them. black plants, pigmented to take in every photon they can, should evolve only on worlds where light is scarce by comparison.
the more light available (and binary stars nearby means A LOT of light), the more of it plants can reject. on a sufficiently illuminated planet, we may even expect to find white plants, desperate to reject and reflect as much light as possible because the quantities are excessive to the point of detriment if unfiltered.