Daniel was killed by a blade, but Treguard's suggestion that they needed a sight potion or seeing-eye spell to survive the CoB was poppycock. So many other teams survived the CoB - Dunstan and Oliver did later in the same series! - without any form of sight. I'm not going to go so far as to suggest that the production team deliberately killed Daniel because he took a red key (which he may well have actually needed as lots of puzzles required keys), but if it transpires that they did, then I wouldn't be surprised.
And Treguard's comment was unjustified. There are other examples, but that's the one that stands out for me.
I think Daniel was in losing status once he had given up the sight potion. He was then given the corridor of blades as it had a relatively high chance of killing him off.