Nigra truo is an adjective next to a noun. It can be any hole that happens to be black. I was surprised to find that this "word" is in ReVo and is used for speaking about the cosmic object named black hole in English. Usually, for speaking about a specific concept, we create a new word like naztruo, butontruo, terpomo... I wonder why nigra truo is different and does not have its own word made from compounding.
The term black hole, when translated into other languages, is basically always either a phonetic transliteration or expressed literally. I do not know of any exception, perhaps because the term is enticingly vivid. Thus, nigra truo is the form most likely to be understood immediately in context.
If you need an unambiguous term, use gravita singularejo.
Well an immediate issue I've noticed with compounding the 2 words is that unlike those other words that you've mentioned, "nigrtruo" is incredibly difficult to pronounce thanks to R being right before T. I can only imagine that this problem alone has a lot to do with it, and not necessarily some grammatical reason. Thanks to Esperanto's agglutinative compounding system however, you could type "nigrtruo" and I'm sure everyone will know what you mean. Saying it is the real problem.
Honestly, the biggest annoyance to me is calling that concept a "black hole" at all. Maybe we could come up with some other term that doesn't parallel the English one. It just feels like there's a better name we can come up with than that. Maybe that compound will work out better.
The proposition ''gravita singularejo'', like nigra truo is a calque (of gratity singularity). Note that ''singularo'' in esperanto only refer to the grammatical number. To translate "singularity", you might probably use unuopo, neordinaraĵo, specialaĵo or kuriozaĵo. ununura and solsola also came to my mind but it seems they only translate "singleton". You could aslso build a word based on rara, like rarano.
As an alternative you can use pezegejo aŭ kosm(a)pezegejo. Here pez/ is certainly suit well, it's already uzed to render the greek root "bary-" of classic terms like "barycentre" (pezocentro). While gravita is in PIV, it still might be interpreted as grav-it-a, a possible collision you will avoid with "pezoforto", that PIV give also have as a synonym entry of gravity.
If you would like to focus more on the "dark side" of the black hole, you should use a word which tells that it's an object which absorb light itself, like lumsorbo or lumsorbaĵo.