The reading of an Elder Scroll is known to make a person go blind unless precautions are made like the moth priests do or temporary blinds if your a Dovahkiin, but the Black Books do not make a person go blind. Not to say that reading a list of mystical books made by a daedric prince of knowledge is any safer since scholars who visit his realm are never seen again. Did Hermaeus Mora ever got his tendrils on an Elder Scroll and decided to experiment on it to see how it works and the black books are just watered down versions of the Elder Scrolls?