[Sawrock]

How about multiple skills:
One three-rank innate skill that gives guard a bonus to magical defense (+10% per rank, still capped at 70% overall).
One two-point passive skill that gives a chance of silence to Prophylaxis (second rank makes it Innate).
One two-point passive skill that gives a chance of silence to Hanging with a status infliction check (second rank makes it Innate).
EDIT: Disregard my suggestions- I'm not for balancing classes evenly for 1v1s and seeing as how it was brought up that yes, Mages are a Black Knight's weakness, I no longer desire to have this class buffed to better combat them.