@Watcher of light and darkness Well this is going to very long and please note that these are the spoilers I read on internet so I don't know much background 😅 So a spoiler at the start of the novel when they get married in this timeline
Spoiler!
Herdine called for a hypnotist to try recover Blair's memories of the night of the fire broke out at Empress's Palace but that memory was so painful for Blair. she cried, retched, unable to breathe properly and so Herdine told the hypnotist to stop the procedure but the hypnotist was stubborn and kept on tormenting Blair mentally right in front of Herdine. Herdine grabbed the hypnotist's collar and told him to stop the procedure immediately and fired the hypnotist.
So Herdine found another way in helping Blair to recover the memories of that night. he brought in a female therapist. every week, she would have an appointment with Blair and asked Blair what was her happiest memories of that time and Blair talked a lot about her happiest memories. in Blair's happiest memories, there was the late empress (Blair's stepmother and Herdine's paternal aunt) as well as young Herdine in Blair's happiest memories, Herdine was about 12 years old if i'm not mistaken (Herdine is older than Blair for sure). the female therapist made a conclusion that Blair has always loved Herdine for a long time, since they were children. the female therapist even told Herdine of this information when he asked the therapist for Blair's progress but he was skeptical because Blair kept on talking about the 1-year contract marriage. (this hypnotist and female therapist scenes happens in the current timeline, the 2nd timeline).
I like to post spoilers for the stories I find interesting because I don't have much time to read novel due to busy schedule
you will find me in most romantic historical manhwa comment sections 
you will find me in most romantic historical manhwa comment sections