Dark in the context of astrophysics means specifically that the object/matter does not interact directly with electromagnetic radiation (eg absorb an optical/microwave/radio photon). So it is probably dark matter, but probably unlikely to be a black hole because we can typically detect a black hole's effects in an indirect manner :P
"dim" implies "there is something normal there that is just not emitting light". "dark" in this astronomical sense means essentially "dim and completely transparent" which is not what you get with e.g. a cold gas cloud - those are opaque.
It's not just that. Remember that in space distance doesn't attenuate electromagnetic radiation. Given perfect line of sight, you could broadcast a 1mW 5GHz signal across the empty space between galaxies and have perfect reception (provided you're very patient)
One also has to consider that at this scale, you cannot have a normal interaction with the EMF and be dim. The normal physical processes of matter at the scale of 1 million suns ends up being quite loud. Black holes that aren't actively eating things form an exception, but black holes aren't normally dark either. Whatever this is it's peculiar, but I wouldn't write-off that it might be an issue with the model they developed for interpreting the data.