If this many people didn't notice the distinction between this and Disrupting Shoal, it's not printable.
As always, context is everything. If there were a cycle of "almost free" pitch spells so that the alternate cost mechanic was something players could see in other ways, I think it could get printed. Additionally, if anyone has suggestions to make the functionality clearer (and seem less like Disrupting Shoal), I would be glad to hear them.
For example:
Blast of Will
U
Instant
As an additional cost to cast Blast of Will, pay X mana or discard a card with converted mana cost X.
Counter target spell with converted mana cost X.
In fact, I think I like this wording better. I'm not sure if it works in the rules, or if the templating is proper, but I'm going to update the current wording to reflect this change.