This actually bugs me about articles on new discoveries: they almost always try to talk about potential applications, no matter how far-fetched they are. Sometimes the most honest answer is that we currently don't know any potential applications.
In this case, we have something that may be similar to graphene, but can only exist at 4000°C and 1.4 million atmospheres. Graphene is interesting for electronics, so they suggest black nitrogen might be useful for electronics too. But of course graphene has the slight advantage that it's relatively easy to produce and doesn't dissolve at temperatures lower than the surface of a star.
In this case, we have something that may be similar to graphene, but can only exist at 4000°C and 1.4 million atmospheres. Graphene is interesting for electronics, so they suggest black nitrogen might be useful for electronics too. But of course graphene has the slight advantage that it's relatively easy to produce and doesn't dissolve at temperatures lower than the surface of a star.