Hey, let's change the 3.x "You may select from the following options" to "You might issue a command other than one described here. If you do so, the GM determines how the target behaves" for 5e, because no one ever came up with cheesy commands in 1e!
Sorry. Anyway, the official party line back in the day, IIRC, was that ambiguous commands would fail. Since "Blaspheme!" is innately ambiguous - do you, as a Cleric, mean blaspheme against your own deity? Or against the targets? What if he's an atheist? - it probably does nothing.