I was reading about the planet TrES-2b which is less reflective than charcoal. What could possibly be its composition?
The article mentions exotic compounds that are not naturally found on Earth (either at all or in the same form), like vaporized sodium, potassium, and titanium oxide. However, since this is explicitly disparaged as not sufficient to explain all of its absorption, we have to search for other reasons.
Honestly, my first guess is experimental error or some unexplained discrepancy that means it's not actually that dark, because it's hard to imagine a gas giant not primarily composd of hydrogen, or completely enveloped in some black substance.
The next most plausible theory I can think of is that there is some sort of preferred-direction optics (like a sun pillar) redirects light towards the planet, very effectively trapping it, or reflecting it in a direction not towards Earth, e.g. towards the planet's poles, making it appear dark.