But a single neuron isn't "aware" of the mind of the brain as a whole.
So a human would never know if the human-race had a hive mind.
Never know about the existence of a hive mind (or hive mind property) is not a correct conclusion here. Humans may or may not recognize that situation, as it is not essential for the functioning of it as a whole(and what that first assumption actually mean). But as you mention humans are more elaborate elements here, and they can recognize the hiveminds of things, at the level of their capacity. What they do not have is the general picture, a look outside the system. And they will be puzzled by so many things how it looks from a position of such human "neuron" as they do not capable of trace all the reasons and hold the representation of the system.
And in general, humans are like neuron layers in a multilayer network, and they are distinct/visible enough as connections with other members of the network aren't so tight(speech, vision, different types of expression, work, a product of work, friends, random people, family). So as connections are more chaotic, humans interact directly or not directly with plenty of people, but few connections are more permanent, so as Dunbar's Number is relevant here.
So any social group, which has some identification token and some set of base ideas around which the identity is built, can be considered as a weak interconnected hivemind, but more often it can be identified as swarm intelligence.
Considering that you can observe interactions between swarms/hiveminds in real life and it takes multiple forms. Negotiations one to one between some members of a swarm are a form of interaction between hiveminds build around some idea/set of ideas in case of humans, or maybe some technology which joins a particular set of humans in the future. Good teamwork, is also can be considered as a manifestation of hivemindness.
So as other actions or exchange of ideas in different forms - media, public speeches, books, movies, art, economics, wars etc - all are the forms of interaction between swarms, their way of talking.
All those we observe today, and there is nothing super mindblowing in that, it just needs to be recognized, that one of the problems you started with.
May it take some different forms, be more exotic to our today's perceptions - yes, why not. If you haven't read Echopraxia and Blindsight by the author Peter Watts - those are novels sort of full of that hive mind intelligence notion, hiveminds of different kinds.
What they gonna say to each other
That is a complex question. Basically, it can be anything, and interaction of base elements of one system to base elements of another system - which you may recognize as communications and understand the sense of those communications, but it does not necessarily mean you recognize or understand the message from one hivemind to another.
In some basic cases, and simple messages you may do, as in those cases hivemind is merely just way to organize base elements, to connect them and the message can be simple in terms of fight or flight response on the level of those hiveminds.
but with a more complex matter, for more complex organized hiveminds(meaning more intellectually sophisticated as hiveminds themselves) it may be beyond the comprehension.
In that sense, the message of one hivemind to another may be anything - starting from simple things to save the universe from death. In that regard religions concept of god and deities, in general, can be applied to advanced hiveminds, and it would be funny if it comes from that source.
Generally, hiveminds are somewhat a reflection of the base elements, so as for naturally evolved life, questions of self-preservation as a hivemind, may be relevant to them, so you may expect it to be one of the subjects they may "discuss".
Depending on resources, and energy for their activity - those also can be questioned to be "discussed".
what is a better way to organize the hivemind - also can be the subject of discussion and testing.
Those are quite basic topics, which basically fit the narrative of some primitive animal, from our standpoint of view. What would it be like a human level of intelligence on the scale, unfortunately, it seems like beyond comprehension stuff, at least at its potential depth. In approximate terms, there will be plenty of stuff we may describe in our terms, but so as the stuff which is too complex. But for to have that too complex stuff a hivemind has to evolve, sufficiently for that, even if there is enough hivemind unconsciousness complexity right at the beginning, as with humans today.
Complexity about which people think for millennia, like 10 thousand years in the past and you sure can find some stuff which is fresh today as it was the day it was born as an idea.
In short, take whatever you like, you just can't miss, by a definition.