Yes, BD will be powdery. A stiff toothbrush (cut down bristles) or a toothpick will brush some away without harming the coin. Under magnification it will look like a
fine powder.
The same colouring (especially in photos) can be found in very hard encrustations that are actually malachite (copper carbonate hydroxide) build up from the copper content. They are very hard and completely stable and safe. They will look like crystal formations under magnification (even magnifying
glass) and won't brush off with toothbrush or toothpick.
SC