Others here have done this much more than I, and will have better advice. However, my personal advice is to go
slow. Start with weaker acids (FeCl is strong stuff). Try wiping on warm citrus fruit juices to see if you get the effect you're looking for. Keep a bottle of base nearby (baking powder dissolved in water works well) to neutralize the acid when the desired etch is obtained.
Soaking for long periods of time can be problematic. (Don't ask.
)
Incidently, Rivkin, what kind of blade is it?