(a) Either the Storyteller likes having part of the challenge be "what does this enemy Charm do?" or not. If not, then Blinding the Boar probably looks like the Sidereal going insanely blurry and hard to focus on, and that effect fades slowly and obviously over five rounds-like when one's eye recovers from pressure on it- so that you can estimate that it will expire.
(b) Blinding the Boar has limited, specialized use. Unfortunately, if you're a Solar who feels that you *have* to be able to fight the Sidereal in single combat by standing there, slugging it out, ignoring the penalties, and not pulling any fancy tricks of your own, *you* are that limited use. You are the boar. You are exactly the circumstance this Charm is specialized to handle. See also Underling Invisibility Technique. :)
Blinding The Boar