A champion fighter im dming leveled up, and getting the extra fighting style, chose Blind Fighting. I ran into this at one of my recent sessions, when an invisible creature attacked, and I completely forgot they could see it with blindsight. I described it just as the hear and sense the footfalls, breathing and movements of larger creature behind them. I didn’t really mention more than that so I’m wondering how you guys describe things to characters with blindsight.

Also how would you describe say, a scrying sensor with this? A bbeg in my campaign is monitoring the party frequently and they’ve almost caught him several times with see invisibility. A scrying sphere is just like a little sphere, that doesn’t make sound or anything. I heavily relied on other senses to describe the invisible creature, so what should I do if this happens?

Thanks for any help!

For a scrying sensor, I'd just describe it as the character being aware that there's some sort of small, invisible thing that gives off neither sound nor smell.  Blindsight just means that your non-visual senses are acute enough that you can pinpoint the location of an invisible creature if it's inside the range and make out general movements (like that it's attacking).  It doesn't let you see details the way actual vision does, so I think your description is fine.  If  the character has already encountered a non-invisible version of what they're facing, then it makes sense to say something like "the raspy breathing and scraping of claws on stone mixed with the stench of putrid flesh and stagnant swamp water immediately lets you know there's a troll threatening you even though you can't see it."

Blindsight doesn't mean you see invisible creatures, that is Truesight. 

Blindsight is exactly what you described - you know the invisible creature's location due to other senses and have no penalty in fighting against it.  If you're familiar with Game of Thrones, the episodes where Arya learns to fight while blind, that is Blindsight.

Quote from DrJawaPhD >>

Blindsight doesn't mean you see invisible creatures, that is Truesight. 

Blindsight is exactly what you described - you know the invisible creature's location due to other senses and have no penalty in fighting against it.  If you're familiar with Game of Thrones, the episodes where Arya learns to fight while blind, that is Blindsight.

Not quite. Here are the rules.

"Blindsight

A monster with blindsight can perceive its surroundings without relying on sight, within a specific radius.

Blind Fighting
You have blindsight with a range of 10 feet. Within that range, you can effectively see anything that isn’t behind total cover, even if you’re blinded or in darkness. Moreover, you can see an invisible creature within that range, unless the creature successfully hides from you."

Blind sight is not just knowing the location of a creature and being able to fight it without penalty. The rules on Blind Fighting explicitly state that you can "effectively see" anything that isn't behind total cover. This means that the character could cast spells that require them to see the target. This means that the character with blind sight can see what clothes they are wearing, what weapons they are carrying, where they are, where they are going and what they are doing.

Blind sight is the equivalent of vision.

Blind sight doesn't give a mechanism for its operation except in the case of bats and echolocation. Other species have some way to perceive that is equivalent to sight within a specified distance that doesn't rely on sight.

I will usually describe it to a player as a sense of perception where they are aware of everything within the radius that isn't behind total cover, including invisible creatures. In fact, the blind sight creature can't tell if something is invisible or not unless they also have a regular sense of sight and can compare the two. A creature with blind sight with their eyes closed just sees everything within the specified radius.

This does mean that an invisible creature can't sneak up on a creature with blind sight at all unless they hide first (since the description indicates that an invisible creature that successfully hides is capable of avoiding being noticed by blind sight).

Thanks for your help! I didn’t realize how strange blindsight was until this just happened, so thanks for your suggestions!

Yeah, blindsight is a catchall for any sense that allows you to see somethings location without conventional sight.  The narrative description you use should depend on the specifics of how they gained that ability and the player can have some input in how their blind sight works. 

Whether they feel the flow of the air, sense the life force round them or have some kind of precognitive spider sense it doesn't really matter. 

Quote from Bybyroro >>

I’m wondering how you guys describe things to characters with blindsight.

Also how would you describe say, a scrying sensor with this? 

 I'd describe a character with Blind Fighting as seeing invisible creatures due to heightened senses or extrasensory perception, like a sixth sense.

Depending on the type of scrying sensor, i'd describe it as vacillating silhouette if direct sight is possible, or extrasensory perception somehow otherwise.

Quote from Solarsyphon >>

The narrative description you use should depend on the specifics of how they gained that ability and the player can have some input in how their blind sight works. 

 This right here. Get the player involved. If they haven't already, have them describe to you how their blindsight works. I don't like to tell people how to play, but if you just say "my character has blindsight" and never think about what that actually represents for that character, you are really missing out on some good roleplaying opportunities. Let them tell you how they see the world.