The blank is an exchange. That is what a ruled line with space above and below means in a document written by a man who bound himself.

CorvinaHe takes a name out and puts a name in. The clause is restored and the page is clean, and the ledger still balances, because he never once let a ledger not balance.

MalfoidSo somebody's name comes off this book and somebody else's name goes on it. Whose?

CorvinaAnybody on the roll, or anybody who has sworn on the book, which is a list thirty people long and most of them are dead.