I believe I read somewhere that silkroad hedges to dollars when the buyer buys, and back to bitcoin when the buyer marks as delivered and they need to pay the seller. If that's true, I'd expect slight downward pressure during the sale, and upward pressure over the coming weeks as people's deliveries arrive.
Only if a vendor chooses to use the service, SR charges a fee to hedge a purchase like this in US dollars on top of their standard fee so not everybody uses it.