Post History

There are perhaps two questions here. The first is whether the planet is intrinsically dark, i.e. it reflects only a small fraction of the light that reaches it from the Sun and emits only a small amount of blackbody radiation. This can be quantified by the planet's [albedo](https://en.wikipedia.org/wiki/Albedo), which depends on the composition of its atmosphere and, if the atmosphere is thin enough, its surface. There are very few constraints on any of its properties, but given its rough distance from the Sun and a reasonable guess as to its mass, it's likely to be an ice giant like Neptune, and as such would have a similar albedo ([Fortney et al. 2016](https://arxiv.org/abs/1604.07424), [Brown & Batygin 2016](https://arxiv.org/abs/1603.05712)). If it's icy and, in particular, at all rocky, it would be unlikely to emit much radiation on its own - the [Kelvin-Helmholtz mechanism](https://en.wikipedia.org/wiki/Kelvin%E2%80%93Helmholtz_mechanism) isn't significant for such bodies.

To recap: It probably won't emit much light of its own, but it's unlikely to have an albedo low enough to qualify as "dark" for any reasonable definition of the word. 

The second question is whether the planet has a reasonably normal albedo (likely) but *appears* too dim to be seen by our telescopes because of a combination of size and distance. Certainly, the further away an object is, the lower the flux of its reflected light, and the smaller an object is, the less light it will reflect. The answer to this is that we simply don't know, because there aren't enough constraints on its orbital or physical parameters. There's also the possibility that it could be detectable in some regions of the spectrum but not others. For instance, [we don't have instruments that can search the far-infrared with large enough fields of view](https://astronomy.stackexchange.com/a/13315/2153) to make an initial detection.