It only makes sense. They are the same exact ability. Only difference is the reward for a successful block, where sivir's gives her unlimited mana on top of costing nothing.
The majority of Reddit seems to agree: https://www.reddit.com/r/leagueoflegends/comments/3j00ji/can_nocturne_spellshield_cost_0_mana/
Nocturne gives a % attack speed boost passive... When he gets a successful block, it doubles..... The rank 5 of it is 40%... Double - 80%..... I think it is VERY well rewarding and SHOULD have a mana cost for something as significant as that
True, Would be logical if they both cost nothing and I like any buff to 
.
Nocturne's gives him an attack speed steroid if he blocks something.
But only sivir's spell shield gives mana.
Sivir also loses mana extremely fast if she is pushing lane. And if she isn't, she's just a speedy marksman.
If Nocturne pressed Q+W and lost almost half of his mana pool, he wouldn't have an early game
