Flavor creates a design dilemma.
On the one hand, flavor is so important for the enjoyment of a mechanical benefit.
But on the other hand, for the same reason, getting stuck with an unwanted flavor can impair the enjoyment of the benefit.
(Here, the question is whether the Hexblade needs to be gloomy.)
A solid solution is offer three vivid flavors, as examples, plus mention the DM might have a different flavor for it (depending on the setting that the DM is running). But then, it duplicates the amount of flavor design necessary for each significant mechanical feature.
Maybe two examples, plus DM choice as the third, is enough as a rule of thumb.
Personally as DM, if a player chooses the Hexblade Invocation, I want the player to cohere it with the overall character concept, in a narrative way.