For me, I think of it as dark because a large sub-bass quantity makes a headphone sound very solid and grounded. The image that always comes to mind is stone scraping against stone - cavernous, massive and shadowy, hence why I end up choosing dark lol.
Certainly the terms are fun, but often times unhelpful. I suspect half the arguments in Head Fi start because the terms are not being used consistently, but this will always occur in any highly subjective hobby where we attempt to describe a sense (sound) with signifiers (language).
Sometimes that is why I think headphone reviews are useful if they are comparisons with other headphones; and this is marginally helpful at best unless you have heard one of the models being compared. This is also all the more relevant considering the purpose of a headphone review is generally assessing the opportunity cost of buying a headphone - ie: not, "how good is this headphone for x dollars", but "am I better off buying headphone A or headphone B with my x dollars".
Then again, some people (like me) also buy headphones for the experiential aspect: ie, they are not looking for the best / relative performance, they are looking for the specific experience that the headphone might give them given the context of the technology, the design, the company history, emotional attachments etc. For instance, I'm particularly interested in the MDR-1R not just because its a good consumer headphone in a competitive category, but because it represents Sony's interpretation of 'the times' in evolving the technology used in the Z1000.
It's like the difference between thinking "I want to watch a film" and then choosing a film to watch because you have heard it's pretty good, and specifically going to watch a film because you know the directors or the actors and you want to see how they approach this fresh new narrative.
In the first case you might be disappointed because you feel you chose the wrong film on the day, but in the second case you might be disappointed in the film, but never your choice.