This actually is a pretty good question.
My theory is that since bettas have sensory organs I'm sure they can tell male from female. Even if the betta is blind, they will know I it's a male or female in the tank with it.
Males and females five out distinctive hormones when in the same tank and I think this is the way they can tell a male from a female when in the tame tank with the same water flow and aeration.
Now if let's say two males are in separate tanks beside each other, my theory is that they will flare and display their fins also create bubblenests even in the sight of other males because bettas build nests not only for spawning, but out of excitement and as a territorial display. This is why some people mistaken the bubblenest only as 'he wants to spawn.'
Originally Posted by Bethydan
I'm wondering if multiple blind males could be safely housed together. Or if there are other cues they could rely on to identify other males besides sight.