These crabs are about as skittish as any animals we saw on the island group, which is to say, almost sanguine. They don't like the feeling of being loomed over and will skitter into shadow if possible; they seem to interpret a person standing over them as a possible predator. If you stand a little distance away at the same level, your presence is tolerable.
Yes, the blue on the underside is indeed this beautiful shade, similar to that of the feet of the blue-footed booby.