What does the spell say that makes you think the intent is obvious?
For my part, the fact that the spell says that you can't see through it, even if you have darkvision, suggests the intent is that it is opaque, because that's what opaque means. I realize you interpret that line differently (even if I'm still unsure exactly how you're interpreting it), so I know you don't find this evidence convincing. I also acknowledge that the wording of that sentence could have been clearer, but the alternative interpretation (i.e. "
only creatures with darkvision can't see through the darkness" instead of "
even creatures with darkvision can't see through the area") is silly enough to be disfavored.
But between that line in the spell, and the fact that the spell description is complete under the opaque interpretation and incomplete under the transparent interpretation (see, e.g., most of this thread debating how to implement the transparent version at the table while giving effect to both the obscurement rules and the visual description of the spell) I think there's very strong evidence that the opaque interpretation was intended. Strong enough that I feel the word "obvious" would have been warranted were it not for the evident debate in this and other threads.
Considering that the inkblot interpretation leads it to being just slightly more than absolutely useless (outside of the Devil's Sight combo) then I'm not sure asking what purpose it should serve in game is really a relative strength of your position.
With the inkblot interpretation all it does is prevent advantage and disadvantage for everyone involved - and make some spells unable to target though that is a bit of a double edged sword.
It prevents those inside from seeing the battlefield, greatly impairing their situational awareness. They'll only be aware of the locations of opponents that the DM determines to be within hearing range. Unless they have a perfect visual memory, they won't necessarily remember where walls and cover are, potentially wasting actions by shooting at targets they can hear but who may or may not currently be exposed, or spending movement unwisely. Readied actions become much harder as those in the sphere can't rely on visual triggers. They also won't necessarily know what actions are being taken by those outside the sphere, limiting their ability to focus fire on wounded enemies or realize that wounded allies need help.
Of course the same will be true for those outside the darkness having their situational awareness impeded with regards to what is going on inside the darkness. That favors the caster of the darkness, who can decide on the fly whether to cast it on allies or enemies depending on whether the change in situational awareness would be a relative benefit or drawback to those in the darkness under the current situation on the battlefield.
How much these factors actually affect play will, of course depend highly on the types of encounters run at a particular table. Large and/or noisy encounters where the battlefield is larger than the current effective range for hearing opponents' locations make the
Darkness spell more valuable. Dynamic combat styles where the PCs and opponents are constantly moving between full cover rather than standing still and slugging it out will also make Darkness more useful, as target selection becomes more important in a dynamic fight.
The effectiveness of
Darkness also depends on the medium in which the game is being run. Tabletop miniatures with a hand drawn or printed map (or a VTT without dynamic lighting) make Darkness less useful, as the
players can still keep their situational awareness of the layout and events of the battlefield even when their characters are blind. With theatre of the mind (or VTTs with dynamic lighting) darkness can impair the players just as much as the characters, making the spell more useful in practice, as players will no longer need to try to RP the lack of situational awareness.
The effectiveness of the spell also depends on play style. If characters and monsters consistently make tactical choices based on RP considerations, Darkness can be more valuable that it otherwise would be, as targets who suddenly lose their situational awareness may be RP'd as disoriented or panicked.
There's no offensive or defensive use of the spell. It makes them blind to you and you just as blind to them.
Loss of situational awareness has immense offensive and defensive applications at many tables. As discussed above, it's possible that between encounter style, game medium, and playstyle, that opaque darkness might have limited utility at a particular table, but that doesn't impede the tactical value of the spell at other tables.
When the darkness that is blocking line of site is so limited on radius that it can be moved through or around in 1 turn - is it actually effective at blocking line of sight?
Due to the loss of situational awareness, targets trying to escape the darkness may move slowly or non-optimally towards the perimeter (e.g. feeling their way along a wall rather than risk running headlong into an obstruction). Additionally, making the targets move out of the darkness to keep their situational awareness may be the point of a particular casting, by limiting the enemy's ability to make use of advantageous terrain.
That's not true. In 5e RAW combat, unless you are hidden (unseen and unheard) then the enemy knows your position.
Only within effective hearing range, and that varies a lot from table to table. At tables with DMs who take into account ambient noise when determining the effective range at which creatures can be located by sound, that range can easily be short enough not to include the entire encounter.
The same purpose it served back in 3.5 e and the Cleric version served in AD&D 1e.
Also, nobody argued it was impossible to imagine back then.
Because the 3.5 spell was explicit about both what it looked like and the mechanical effects, so there wasn't anything to discuss. (Also, in my experience, no one actually used the spell, opting instead for the opaque variant from one of the supplements.) 5e
Darkness under the transparent interpretation has descriptive and mechanical holes the DM needs to fill, hence the discussion in this thread about how to do so.
