Dark hair started relationship with blonde to use it as writing material, caught real feelings but got found out for using him. Blondie also got blackmailed with photos of them by some family member, so he had another reason to end the relationship.
It was more of an acquaintanceship turned friendship, then sort of fuck buddies, resulting in a romantic relationship, if I remember correctly. Juheon misunderstood the situation and Dohu's interest. Dohu was never looking for things to get sexual nor romantic in the beginning. He just wanted a certain personality to observe. And since he sucks at reading the room a lot of the time, he didn't see the harm in using his experiences with Juheon as material for his writing. He should have said something, absolutely, but I also think Juheon arrived at certain conclusions that dismissed what he had learned about Dohu's personality (and possible autism?). As always, communication is key. They both fucked up a bit. I do understand why Juheon felt betrayed, though.
Read this a long time ago. Y did they broke up again?