I think this story did a reasonable job of setting up that black hair is not really the long lost friend of her girlfriend, between the title and the photo conversation at the beginning. I also love the art as always with this mangaka. But I felt the "she's a secret psycho murderer" twist was absurd and made it difficult to connect with the story. I think it would have worked better had black hair just found the phone of blond girl in the street, or heard the story from her before she moved, or found out through some other non-violent way. Then I might feel pity for a girl who wants so desperately to have a fairy tale romance that she'll steal someone else's. And there would be an ambiguity to the non-ending of them just being happily together, since you never know when real blonde girl might show up. As written, I just feel miserable that a murderer got away with murder.
I don't mean to say that the author needed to do exactly what I describe; there are a lot of interesting ways you could take a story about a girl who pretends her love story was more romantic than it really was. I think anyone who met their partner through online dating, a bar, or some other common method can relate to the desire for a more magical story. But when you make that character not only deluded but a murderer, it stops her from being relatable. So even though I think this is a neat premise, the conclusion pretty much ruins it for me.