Just started playing Into the Void. Warming up against computers. I noticed a blue target in a wide area around my units while I played against Terran. I'm thinking it's the Liberator but I recall using that unit and it's defender mode sat there and attacked units one at a time. Not sure what ability I was seeing.
You are correct it is the Liberator's attack circle while in defender mode. See here for image.
The colour is the same as the player's colour, that's why in my image it is green. In your case your opponent must've been blue and thus the attack circle is also blue.