They aren't really blind. It's one of the bat myths. Their eye sight can be poorly developed since they are mostly active at twilight and night. They find their prey (insects or fruit) by emitting high-pitched sounds and listening to the echoes. So, it could have been very easy for Angelque's bat to find Barnabas and then send signals about Barnabas back to her.
It's a similar technology that whales and dolphins use or humans in an ultrasound machine or ship sonar.
Wikipedia has an interesting article about sonar and how bats use it. Medical ultrasound is really interesting. Last month, I saw my heart in an echocardiagram. All is well. I just have a slight mitral valve prolapse in one of my chambers aka heart murmur. It makes a hiccup kinda of sound when you hear it. It's so way cool to see it beat in a three dimensional picture. I can only imagine how cool it is for bats to do that type of thing and how easy it makes it for them to find their prey or not bump into the wall.