That would depend if they block light or if it just travels right through them (ignore the fact that in any case anything done that would prevent a reflection would also prevent direct observation). Given the popular portrayal is mirrors do not just have a black silhouette of vampires, I think it would just travel straight through.
Something has to be there, either we see through them (same as the wall behind), or a silhouette filled with something else like pure black or madness inducing colors never seen before.