White, ivory, gray, and black would probably be your best options for blouse color. A silvery gray like this or a black with some sparkle to it like this would be lovely. I would categorize the JSK as being fairly evenly split between Classic and Sweet so you could definitely get more Sweet elements in there with accessories but I also think it would be a mistake to try drowning out the elegance of the Classic elements because they’re really what makes the dress. I think navy or silver shoes would be the way to go. Silver would be great especially if you really played on the silver elements of the dress by tying it into your legwear as well, maybe with black or navy tights that have some sparkle to them which would also add a fun, sort of Sweet element.
I think you could probably get away with that. I would definitely consider working some lighter color, probably that blue, into the blouse area to lighten it up so it isn’t so top heavy. Perhaps you could do a blue neck bow with the black blouse and then maybe incorporate both the black and the blue in your headwear. Then you could draw on the colors from the print for smaller accessories and maybe incorporate them into legwear too, perhaps with black tights or socks that have a light design on them.