I'm going to respond with my standard disclaimer: "It depends."
We have fifty states, each with its own laws that have their own definitions. On top of that, within many of those states there are counties and/or municipalities that have laws, statutes, or ordinances that govern the discharge of firearms. To answer this question would require knowing the jurisdiction, and then reading the particular statutes and/or ordinances that apply in that jurisdiction.
As a general rule, I suspect previous posters are correct in that, more than likely, "discharge" is not well or properly defined, and thus any laws prohibiting discharge of a firearm would include firing blanks.
As to starter pistols (which have plugged barrels and cannot eject a projectile) -- now you also have to review the laws for the definition of "firearm" to determine whether or not a starter pistol is considered a firearm in that jurisdiction.