Question: Bats and Whales, and some monsters like the Hook Horror, have blindsight based on hearing. You can tell it's based on hearing because they also have the Echolocation trait that says their blindsight does not work if they are deafened. For the purpose of this discussion, please assume a totally dark environment.

Beyond the range of their blindsight, these creatures can use their mundane hearing to detect other creatures, and they can benefit from Keen Hearing, giving them advantage on Perception checks that rely on hearing. Within range of their blindsight, what changes? Do they still benefit from Keen Hearing? And how does blindsight change what they can perceive over and above mundane hearing?

In particular, to what extent can they pinpoint the exact position of creatures within and outside the range of their blindsight?

Answer: 
**Within the Blindsight**: They act as normal, as if they see the target. If the target is blinded (by the darkness) they get advantage on their attacks.

**Outside the Blindsight**: They are Blinded. 

> 
> * A blinded creature can't see and automatically fails any ability
>  check that requires sight.
> * Attack rolls against the creature have
>  advantage, and the creature's attack rolls have disadvantage.
> 
> 
> 

This however depends on the creature pinpointing the correct position and as far as I know there is no clear rules on how to do that - although a Wisdom(Perception) roll with advantage from Keen Hearing seems reasonable against a suitable difficulty. 

It is also debatable if the creature grant advantage against projectiles as they need to pass through its blindsight radius before striking, but it seems RAW this is the case.
If the creature they want to target is outside their blindsight range and *not* hidden, they can target them with disadvantage from being blinded. This is not something particular to Keen Hearing, anyone can do this.

You appear to be making a mistake in thinking that if a creature cannot be seen then the default state is their location is unknown - in fact the default is the opposite: the location of a creature is known *unless* they are hiding. See [What advantages does hiding have?](https://rpg.stackexchange.com/questions/63441/what-advantages-does-hiding-have/63453#63453)

If they are hiding then Keen Hearing allows advantage on the Wisdom (Perception) check to beat their Dexterity (Stealth) check. Normally, this is passive versus active so advantage gives a +5 but if the perceiver wants to spend an action they can make an active check.