This might sound like a foolish assumption on Seward's part, given the popular conception that bats are blind. In fact, all bats can see, although some species can't see in colour and their sight is not strong in daytime, hence the old saying "Blind as a bat".
Sight is not as important as echolocation for many species of bats. By emitting a series of ultrasonic noises and detecting the reflection of those noises off surfaces in their vicinity, bats can form a remarkable 3-dimensional impression of the placement of insects, trees and other objects around them, even in total darkness. Echolocation was first described in 1940, around the same time Radar was being developed, so Bram Stoker would not have known of this further bizarre attribute of Dracula's favourite creature. However he might have come across some early research into bats suggesting a mysterious "sixth sense", including an eighteenth century experiment showing that a bat was able to navigate around a dark room much more successfully than an owl - unless one of its ears was blocked.