Originally Posted by
Reality
The MCU Terrigen works exactly the same as the comic book Terrigen. The first examples of it were just made from a melted down Diviner, which mixed the properties of the metal (lethal to non-Inhumans) with the Terrigen itself (transforms Inhumans, does nothing to others). That was remedied when it entered the ecosystem and the Diviner metal was filtered out when the crystals dissolved, leaving the metal to sink.
So Black Bolt had every reason to think that releasing the Terrigen was only going to transform latent Inhumans. It has no history of harming others, aside from some US experiments on soldiers that weren't latent Inhumans and couldn't take the transformation. That it has started hurting mutants is strange, and appears to be new. I reckon something happens in the eight month gap to poison it to mutants, likely a villain's plot. The Skyspears weren't there before, after all.
Other than that, you're right. The two situations are very similar. I really look forward to seeing Cyclops and Black Bolt interact because of it!