Bocchi the Rock! Creator Picked Hitori's Black Les Paul by Image Search
The detail that Hamaji chose Hitori's signature guitar through an image search, not instrument expertise, shows how the manga's authenticity came from learning the culture rather than prior knowledge.
Reporting from 1 source: ASCII.jp.
ASCII.jp reports that Bocchi the Rock! creator Aki Hamaji knew almost nothing about guitars or bands when the manga began, and chose Hitori's black Les Paul Custom by searching for "black" and "guitar" after setting the character design. She had never visited Shimokitazawa or a live house before drawing the manga, and learned the culture as she went.
ASCII.jp's Kindle sale column includes a profile of Bocchi the Rock! creator Aki Hamaji, revealing that she could not tell a guitar from a bass when the series was first planned. Hitori's black Les Paul Custom was chosen after Hamaji set the character design and searched for "black" and "guitar," picking a top result.
Hamaji had never been to Shimokitazawa or a live house before drawing the manga. On her first visit, a staff member asked which band she had come to see; she said none and got a puzzled look. After serialization began, she kept studying and interviewing, working those findings into the story's trivia.
Synthesized by Yomimono from the 1 cited source below, including Japanese-language reporting where cited, then editorially reviewed before publishing.