Does Blindsight detect a scrying sensor?

Question

My party's going to be encountering an Elder Oblex that's gained more sentience than usual. The party has a mirror of Scrying (cast once per day without components/slots). They're wanting to use said mirror to spy on the Oblex, get an idea where exactly it is, plan an ambush for it.

Elder Oblex has blindsight. Assuming the Oblex fails the save to resist the Scrying spell, would it notice the sensor created by the spell? The sensor is invisible, which would be ignored by the blindsight, but I'm not sure if the sensor does any other kind of sensory effect that could be detectable by blindsight.

The exact wording of Scrying is:

On a failed save, the spell creates an invisible sensor within 10 feet of the target. You can see and hear through the sensor as if you were there. The sensor moves with the target, remaining within 10 feet of it for the duration. A creature that can see invisible objects sees the sensor as a luminous orb about the size of your fist.

Answer 1

Blindsight can detect invisible objects.

A creature that can see invisible objects sees the sensor

Implies that this sensor is an object or at least treated like an object for the matter of perception.

Blindsight (PHB 183):

can perceive its surroundings without relying on sight

Blindsight doesn't inform the creature about the nature of the spell. It merely detects that there is an object. As a special sense Blindsight bypasses the mechanics of being invisible, the condition states (my emphasis):

An invisible creature is impossible to see without the aid of magic or a special sense.

Blindsight is a special type of sense, and while the sensor is no creature, the logic applies to objects as well.

Answer 2

Would Blindsight see it? Well, maybe.

The description of the spell simply says the sensor is invisible. Invisible objects still have mass, physical dimensions and the like. If other properties were to be applied, terms like 'intangible', 'ethereal', etc would have been used. So it is just invisible going by Rules As Written.

The sensor is merely described as:

A creature that can see invisible objects sees the sensor as a luminous orb about the size of your fist.

Going by that description it gives off no sound or smell or other detectable evidence of its presence beyond being a glowing ball if it can be viewed.

This part - "A creature that can see invisible objects sees the sensor" - also gives us a slight preference for claiming the scrying sensor is actually an object, but it isn't a water tight argument.

Blindsight is described as:

A creature with blindsight can perceive its surroundings without relying on sight, within a specific radius. Creatures without eyes, such as oozes, and creatures with echolocation or heightened senses, such as bats and true dragons, have this sense.

So it would seem that the type of blindsight is important. Echolocation would detect the sensor's presence (as it has a physical manifestation) but, as worded, heightened senses would not unless you decide the sensor does give off some form clue to its presence that could be detected by Blindsight.

So, what would Blindsight actually mean for an Elder Oblex? Going by the stat block of the Oblex I would say not. It has no abilities to suggest it could detect a silent, hovering, invisible ball