When you induct a new member into the Blades, it seems that their outfit gets replaced with the Blades Armor outfit.

How is this done? Can it be done? I was wondering if it were possible to replace Brunwulf Free-Winter's outfit with a more upperclass outfit instead of his armor after he gets anointed as Jarl of Windhelm.

http://www.creationkit.com/SetOutfit_-_Actor should do it.