I was reading through the Secret Ansem Reports in KHII a few minutes ago, and in #3, I noticed something that may point to why Riku wore the blindfold.
"It appears my other five apprentices, believing it was for the sake of research, stared deep into the darkness and were pulled into it"
(Forgive me if that's a bit off.)
If that's to be taken literally, then Riku might have put the blindfold on so he wouldn't be pulled into the darkness and take Xehanort's Heartless's form. Then when he took it off while fighting Roxas, he was pulled into the darkness, and we know what happened after that.
Sorry if someone's already posted a theory like this, just sharing what I found.
"It appears my other five apprentices, believing it was for the sake of research, stared deep into the darkness and were pulled into it"
(Forgive me if that's a bit off.)
If that's to be taken literally, then Riku might have put the blindfold on so he wouldn't be pulled into the darkness and take Xehanort's Heartless's form. Then when he took it off while fighting Roxas, he was pulled into the darkness, and we know what happened after that.
Sorry if someone's already posted a theory like this, just sharing what I found.