Cast Fog Cloud, then wildshape into a giant poisonous snake. You have blindsight to 10 ft and 10 ft reach on your attack. Anyone you're fighting in the cloud has blinded status, so you have advantage on your attacks plus big damage if they fail their con save against the poison and they have disadvantage on their attacks if they manage to find you (since they're trying to find something to attack that they're not able to see) and you have an ac of 14 in that form, one of the highest natural armor acs you can have without being a moon druid. And if you find an eversmoking bottle? Conjure animals and have 8 other Giant poisonous snakes in the 120 ft cloud centered on you with you, doing the same thing. Get Rekt.
Well, sure fog cloud is a good tactical thing. But remeber that being blinded does not mean that the enemies do not know where you are. Only if you succefully try to hide they do not know where to hit. Yes they still have disadvantage on attacks roll.
Also, a moon druid can do the same thing, even better.
"Combatants often try to escape their foes’ notice by hiding, casting the invisibility spell, or lurking in darkness. When you attack a target that you can’t see, you have disadvantage on the attack roll. This is true whether you’re guessing the target’s location or you’re targeting a creature you can hear but not see. If the target isn’t in the location you targeted, you automatically miss, but the DM typically just says that the attack missed, not whether you guessed the target’s location correctly. When a creature can’t see you, you have advantage on attack rolls against it. If you are hidden—both unseen and unheard—when you make an attack, you give away your location when the attack hits or misses."
Based on the above from the phb, I would think the person can attempt an attack based on hearing, but it would require a perception check(which they would automatically fail if they are deafened) to target the correct square before being able to even make the attack at disadvantage. If they can't find the correct square, they would be making a random shot at one of the squares within range or spending their movement scouting around to try to find the target. I would give them advantage on the perception check if the snake attacked them since they heard/felt where the snake was coming from, but since it's making the attack at reach, it could have moved after without the target's knowledge. If it was in melee range, I would agree with you that it would know where the target is located and just makes the attack at disadvantage because they can't see it, but without making a perception check to find it.
Which is why I said you would only use conjure animals if you have an eversmoking bottle which mimics fog cloud but without needing to cast the spell or concentrate on it and at a much higher level.
I specifically stated this was for if you are not a moon druid. Moon druids can do other things, but this lets a primary caster get into it if necessary. That said, the same tactic can be used by a moon druid if they wildshape into a creature with blindsight and reach, like the giant constrictor snake at lvl 6 when they can wildshape into a cr 2 creature.
Yes, the location you were, not necessarily the location you are. If the enemy can't see you at all because they are blind, you attack, and then you move, they know where you were but not necessarily where you are, at least not without doing a check. I would let the person make this check as part of their attack if they are making a ranged attack or use their movement to scout around the immediate area do so if they are going to attempt attacking from melee. Close your eyes and shoot a gun at a target 10 feet away without knowing where it is. What are the chances you hit it? Even if you know where it is, you have a decreased chance to hit it at all or where you want to. Forcing a check just makes sense if the target isn't right next to you.
I see your points. I just wanted to point out the official rules. In close combat, to be hidden means to be unheard and unseen. if you are unseen but heard, you still need to take the Hide action to be completely hidden. Of course, if you are unseen you always have the advatage for the attacks rolls.
I think the rules are a bit conflicting. There's the rule for darkness and visually obscured things from dim light, total darkness, lightly obscured, and heavily obscured, and seemingly conflicting separate rules for unseen attackers.