The Bork-bork will actually outdamage Bludthirsta when you whack someone with over 1500 health. Against a lower-health target, BT wins out. Building crit tilts the advantage towards BT, but since nobody but carries build much crit at all, it's not terribly important for bruisers. Take that how you will.
Bloodthirster is a stronger item if you have strong AD ratios, though Fiora really doesn't, when her only AD ratios are her rather unspectacular Lunge and her ult, which procs on-hits anyways, and is basically a modified autoattack of sorts.
I think Fiora would actually benefit from Hydra as a third big AD item, after, say, Cleavage and Bork-Bork. Her wave clear isn't phenomenal, so with it she'll be able to push out quickly, which is always useful once you're no longer laning.
Also Hydra and BC means she'll have more cleavage in her build. please don't hurt me please don't hurt me