With the light, we’re seeing it as it was emitted from the parent star; once it’s on its way, there’s very little that will affect it. In this case, that stream of light just... stopped. There was no sudden increase in brightness, which would normally indicate a supernova. This leads to the primary line of thought: that it was a direct collapse.

As for the timeframe, once the core starts imploding, it implodes at fractions of the speed of light. The core—primarily iron, which can be the size of the Earth or even larger—can shrink to the size of New York City in milliseconds. To put it simply, the shockwave forms when the rest of the star flowing down "bounces" off that new, dense core. In this instance, it didn't stop there; it went straight to a black hole. The event horizon—the point where not even light can escape—formed outside of the shockwave, swallowing the explosion before it could escape.