This has just happened to me too, I never sell (or deliberatly drop) letters, notes etc. I found the blood but no longer had the spell
Fortunatly the game makes a lot of saves, I found one a few hours after I did the mines, the item was missing then but less of the zone had been explored, I found it after an hour of searching out side the inn at Silverglen, no idea how it got dropped, items like this should be labled "quest" and unsellable/droppable.
