The Alexiad claims that blinding involved applying a red hot poker to the eyes and then gouging out the charred remains. (Source: Book 1 of the Alexiad) Do note that this isn't described explicitly though, so I may very well be wrong. Anna assumes the reader know how people were traditionally blinded and only describes how a fake blinding deviates from standard procedure.

Edit: Specifically, she says that the red hot poker didn't actually touch the eyes of the man to be "blinded," and that the man responsible for gouging the dude's eyes out only pretended to do it. Afterward, bandages were applied to make it look as though his eyes had had been removed.