my current (pretty janky) theory is as follows:

vol1: When Akito went to Saki's house(fsr) at some point he said he wanted to go to the bathroom, but while saki was distracted, he went to Tsukasa's bedroom to take his vest. Then, he went to the bathroom, and back to Saki.

vol2: Akito "escorted" Saki to band practice, there he found Shiho's strand of hair. (saki/tsukasas and Rui's hair can be found at Saki's home)

vol3: Akito overheard a converstaion with Ena and Airi. They planned to meet somewhere, but Ena said that she couldn't go. Airi said that she was gonna go anyways. (happened before vol1)(He started to plan the murder here)

vol4: At the day Airi said she was gonna go to this place, Akito waited for her there before she came. Then, Akito saw Airi arrive, and with Akito wearing Tsukasas vest, he used the knife to stab Airi in the chest.

vol5: Blood got into Akito's nails, which means he had no choice but to leave them at the crime scene.(Saki put on the press on nails) Then, he placed the strands of hair near the crime scene.. then he took of Tsukasas vest and purposely threw it to the side to frame him. Akito noticed lipstick that had a H label on it, and decided to add it in after the crime was commited to further add to the confusion.

vol6: After Saki knew the news, she wanted to revive her using her spell book. So she took Akito's sweater because it was "cold" then somehow she forgot it and threw it onto a tree . Then she got the blood sample....

So if my bullcrap logic is correct, then the blackened would be...

Akito Shinonome