Arch blanks means:

It is important to keep in mind that neither the ligation method nor the prescription adjustments placed in straight-wire brackets have anything to do with arch form, which is still established by the shape of the archwires. Arch form is particularly important  during the finishing stage of treatment, when heavy rectangular archwires are employed. Preformed archwires are best considered  as “arch blanks” and sometimes are listed in the catalogs in that way. The name is appropriate, because this properly implies that a degree of individualization of their shape will be required to accommodate the needs of patients. Contemporary orthodontics , Proffit 6th ed Page no 343