i think the reason it was not yet brought up is because iruma himself did not know his blood can heal demons, and although there are some who knows that iruma is a human, i guess it was not known that humans' blood can heal demons.
I agree!! i really wanna see how they will react when they found out the fact that iruma's blood! (given that the demons will not go into frenzy) but i think they will be using this as a build up for iruma finally becoming the demon king, especially the prophecy mentioned that the next demon king possess a blood with healing powers

I hope they bring back the human blood thing, I think it’s an interesting aspect that iruma doesn’t use enough! excited tho