My first thoughts on this article is that it would indeed take the mass of a star being converted into energy to see from a great distance, a blue flash with such intensity, radiated in a spherical fashion… enough energy to irradiate the galaxy from which is from lifeless.

Personally I do not think they take into consideration that a black hole would form a wormhole with any passing star. Like a frog who’s tongue flicks out and grabs a fly.
A black hole would suck a nearby star dry through the wormhole occurring in the same “timeframe” as the black hole, which is essentially taking forever because time has nearly stopped. in other words, no bright flash because of the time constraints metering out the energy slowly at a lower wavelength… This also keeps the energy blast from sterilizing the galaxy from where the absorption occurs.

A collision of a black hole and a star, on the other hand, would result in the mass of the star collecting on the transition zone event horizon unable to merge because of time constraints, but unable to resist the gravitational pull. The stars mass would surround and cover the event horizon and appear as a quasar until it’s mass can slowly be consumed.

So what would be my hypothesis on the blue flash?
A magnetar/neutron star would have tightly focused jets from their poles. Like a Mazer beam that creates x-rays and the blue flash along its path of travel, even to distant galaxies. The likelihood that you would encounter a tightly focused beam like this is very slight, and very rare. Considering how many stars in the universe, the occurrence happens but cannot be predicted.

Here’s an example.
https://www.nasa.gov/universe/stars/neutron-stars/magnetars/a-star-that-bursts-blinks-and-disappears/

The Uncertainty of Science™. Applies to the hard and soft sciences. A very expensive direction of resources, because some people can’t admit that.