I liked this fic a great deal. Snape's characterisation, I think, was one of my favourite parts; he's still Slytherin to the core, but he has an edge of complexity that fascinates. You have a wonderful way of making him sarcastic but not pretentious, caring but not insipid (his letter to Harry touched me more than a conventional love letter would have).
You also crafted this story cleverly: the clues lie in wait, and the little peculiarities -- Snape's obvious physical deterioration, his dubious potion-making, the blindfolding -- make the mystery's conclusion believable.