I think they are not necessarily nice 'cause I remember where Lucas asked suspiciously whether the mermaid pearl belongs to her in front of everyone during a lunch and if wechoose to say yes then everyone says "you're one of them", they actually send you to a dungeon or something where one of the guards she recognises as Elias and meets her fate and it implies that this is what happens to merpeople if they're found by the kingdom(I think later it's even implied when mc is caught in the water tank that everyone outside palace thinks that She's been hanged).

people acted prejudiced even when Elias and Ondine were convincing that merpeople and humans can coexist in end.I felt they gradually warmed up to the fact that mc is a mermaid and got over their prejudices. I actually feel that the whole bloodlines thing could be just about the relationship between merpeople and humans as a taboo especially as the whole story culminates with the merpeople from the sea and humans from land finally returning to each other now that what divided them has been nullified. But maybe I'm too reading too much into it.

IDK event blurbs generally are objective facts like it's for Shadows of London or HYSC but this story is fantasy and the facts are made up fantasy elements and given that the story views the relationship between merpeople and humans as taboo and in many endings , the mc ends up .I think, being hanged if discovered earlier in the story, I think it can be viewed as being against mixing with the sacred pact mixing. But it is still a weird choice of words.