From my understanding is that.
Eunha's soul was supposed to go into Beatrice's body, but somehow with black magic Raeliana blocked Eunha's way and Rae's soul was transported to Beatrice's body. Hence Eunha's soul searched an empty shell, which is Raeliana, our current MC.
What the black priestess means by "the Blonde is your fated one, kill the Brunette." is that the original brunette—Raeliana as Beatrice— was Noah's target not literally our MF and Eunha— who is supposed to be Beatrice but instead gone to Rae's body— was the fated partner. She was literally pointing out to kill Beatrice and marry our MC.
Haven't you noticed, this was the antagonist-Raeliana's miscalculation. She thought if she transfers her soul to Beatrice's body, she'll be the main character but what she didn't expect was Fate to be cruel to her. No matter what she do, Eunha/Beatrice will always be Noah's dated partner, no matter which or who's body she transported to (obvs except a man's because this ain't yaoi). That's why Antagonist-Raeliana was desperate to kill our MC.
Hoping I helped you.
You helped me a lot. I was majorly confused for several chapters.
No probs. (๑•ㅂ•)و✧
I'm so confused. This body switch into thing is insane. What I'm more focused on is this plotting between both girls. Like good Lord we do things different in real life. I'd be throwing hand's with anyone that tried to kill me. Your face will be acquainted with the dirt and the street.