Wow, I hadn't heard a thing about this place until I happened upon it by chance today while wandering the Rift. Bethesda really did add a lot more than they were letting on with Dawnguard. I had already shed my vampirism so I can't use the Bloodstone Spring without getting stupid, but I'll bet it's useful for vamps.

What's really strange about it is that this "quest" doesn't show up in your journal at all. Not even in the Miscellaneous section.

Its because its part of the vampire lord questline, you go there for Harkon. Its not hidden, you just didnt do the questline for it.

That makes sense, I guess. I'm surprised a Dawnguarder would have that much access to it.