Traditionally the blind fold was given to certain students that were scared of the opponents arms during chi sau. This would lead to them sticking to them and/or going to the limbs rather than directly to the center with the punch. They would become frightened and in extreme cases move the head away and close the eyes which is not good for fighting. This is a natural response and one has to be trained away. Taking the vision away theoretically would help the process of attacking.
It's part of a lat sau jik chun training exercise. In the context of previous "discussions" I wasn't sure so had to ask much to the delight of LT's love child (BPWT) who seems to want to use it against me all the time.
I never had that problem so never needed a blind fold and it wasn't until I asked PB that I got more information on it. Until then I just thought it was a BS exercise which actually these days in most lineages it is!
"Ving Tsun is a horse not everybody can ride"
Wong Shun Leung.