Frodo's face drew a blank--the sort of blank when one's features cannot even begin express one's thoughts, so the just end up giving up. If anything, he just went a bit paler. "A dream, you say?"
In Frodo's own dream, he had been accompanied by a man with a bow and arrow, but this was not the same person, which he realized with some disappointment. Still... "Your name's not Legolas, is it?"