AoE Blindness Adv/Dis negation?

Quote from Squirrelloid >>

My presumption is that creatures are undetected until positively detected, which is mediated by perception (both passive and search actions).

 Creatures are automatically detected when they take a wide variety of combat actions, including attacking, and once detected, remain detected unless they do something to change that fact. There is no listed combat action that can change your status from "heard" to "unheard".

A character who has never been detected at all cannot be attacked, except by accident.

Quote from Pantagruel666 >>

Quote from Squirrelloid >>

My presumption is that creatures are undetected until positively detected, which is mediated by perception (both passive and search actions).

 Creatures are automatically detected when they take a wide variety of combat actions, including attacking, and once detected, remain detected unless they do something to change that fact. There is no listed combat action that can change your status from "heard" to "unheard".

 I mean, I agree, there's nothing that can change your status from 'heard' to 'unheard'.  But that's because there is no status 'heard' in the rules.  We're into houserules once we start dealing with hearing at all (except for deafness preventing you from hearing at all). 

The word 'heard' occurs 5x in the SRD, two of which aren't about perceiving with hearing at all.  One is about whether you've heard of something.  The other is about the raven being able to mimic any sound it has heard.  The three remaining occurrences are in item descriptions, two of which only tell you how far you can hear them (bell, signal whistle).  Berserker Axe asks whether a creature can be seen or heard by you.

"Hear" occurs more often, such as bardic inspiration (which can be used on a creature within 60' who can see or hear you, suggesting hearing is not guaranteed at 60').  Most of the uses are in spell descriptions (which do not have mechanics for determining what is heard, but do say things like 'you can see through its eyes and hear through its ears as if you were in its space'... whatever that actually means).  There are a bunch of things which ask you to evaluate what can be heard or if something can hear something else, but there's no rules for actually determining that.

In short, the rules never cover hearing besides telling you deafness means you can't hear anything.

Quote from Pantagruel666 >>

A character who has never been detected at all cannot be attacked, except by accident.

The rules actually say you can attack a square.  That's not an accident if you 'find' an undetected creature by hitting it that way.  It would be insanely lucky.  I mean, the DM shouldn't be doing such a thing, since he is omniscient in the context of the game, but a player might.  Although I'd be super impressed if one did.

Quote from Jaysburn >>

Actually in 5.5e it is reasonable to assume that Total Cover does, in fact, block the Darkness spell. Per the rules for Area of Effects: "If all straight lines extending from the point of origin to a location in the area of effect are blocked, that location isn’t included in the area of effect. To block a line, an obstruction must provide Total Cover."

The 5e version of Darkness included the line "spreads around corners," but this was removed from the 5.5e version of Darkness. The word "spreads" is not really enough to override the rule of how AoE's, well, spread.
Many spells had that terminology - "spreads around corners" - in 5e, and it was removed from them in 5.5e (just look at 5e Fireball vs 5.5e Fireball)
I believe this was done intentionally to improve the benefits of being in Cover, as Total Cover not blocking spell effects made trying to be behind it pretty lackluster and pointless in 5e.

 How exactly would you play this with an ongoing effect like Darkness? Say I'm behind a large tree that gives me total cover from the "point of origin". For the duration, is there a tree-shaped area of "normal" (bright or dim, whatever the situation was at the time Darkness was cast) light extending to the edge of the spell? This doesn't really make any sense. 

The way Cover works is spelled out in the rules. Total cover means you "cannot be targeted", Half Cover grants +2 bonus to AC and Dexterity saving throws, and Three-Quarters Cover  grants+5 bonus to AC and Dexterity saving throws. For Darkness, there's no "targeting" or Saving Throws. So comparing it to Fireball is a non-starter. Fireball is an instantaneous effect that forces a Saving Throw that the rules of Cover can interact with. Darkness does not. It also says it "fills" the sphere. Pockets of normal light caused by cover would not satisfy the wording of the spell. 

I'd love to know if you actually would rule this way in your game (that cover provides pockets of normal light within the Darkness spell radius).