The Bluejay was a fictional robber invented by Fenoglio in the Inkworld. He is known to always be wearing a mask of blue feathers, and to have a scar on his left arm. There were many stories that Fenoglio told about the Bluejay (all about trying to get revenge on the Adderhead), but what he didn't realize, was that the stories about the Bluejay were coming true each time he told them. He found out that it was the The Black Prince with a mask, that was committing all the crimes against the Adderhead. Of course, the Adderhead got angry and started searching for the Bluejay everywhere. Once Mo got to the Inkworld, the Adderhead's men captured him, because they mistook him for the Bluejay. Fenoglio had modeled the character of the Bluejay after Mortimer (the hair color, the scar; everything), therefore Mo was brought to the Adderhead's castle (The Castle of Night) and questioned.
Everyone thought that Mo really was the Bluejay becuase, in a way, he was. That's the way Fenoglio wrote him. When "the Bluejay" was captured, he convinced the Adderhead that he would be able to make him immortal. Meggie lied to the Adderhead, and told him that Mo really was immortal, too, so that was proof that he could do it. The Adderheadasked them how, and Meggie told him that her father would bind him a book full of 500 blank pages, and that the Adderhead would have to write his name on the first page. From then on, he would be immortal, unless the book was either damaged, or three certain words were written in it: heart, spell, and death. So Meggie and Mo made a deal with the Adderhead. The Adderhead said that if they could make him immortal with this book then, in turn, he would set Mo free. Mo and Meggie's part of the plan was not only to bind him a book which he though would make him immortal (and also buy them their freedom), but prove that Mo really was not the a robber, and that he was truly a bookbinder.