Scientists may have just discovered a new class of black holes.

First of all though: I'm not quite sure where you're going with your question. What is it that surprises you about the mass of that black hole in relation to fusion fueled stars? Keep in mind that technically ANYTHING could become a black hole. The important thing about stars is just that you need a specific initial mass for the star so that it leaves behind a black hole in the end and it's only the core that collapses into a compact object; most of the shell is gone by the end of a star's evolution.

To answer your question about density: Main sequence stars, those that turn hydrogen into helium in their cores, have an anti-proportional relationship between their mass and their volume, meaning that the more massive a star is the less dense it is if you sum up their whole mass over the whole volume; that last bit is important, because the cores of more massive stars are denser than what you find the sun simply because there's a bigger shell pressing down on it, but the shell, which is what mainly contributes to both mass and volume, is way less dense in massive stars compared to, for example, the sun.

The reason the shell is less dense is that the energy being released in the core counteracts gravity, and is distributed throughout the star. So, if you have a more massive and dense core that can produce a lot of energy, that energy heats up the shell and spreads it out, the same as how most fluids and gases become expand (= become less dense) when you heat them up.

Just quickly about massive stars leaving behind black holes: When a star leaves behind a compact object, be it a white dwarf, a neutron star, or a black hole, what constitutes that object is what remains of the core(!) of the star, not the star as a whole. During a collapse, in most scenarios, it is the core that collapses into some compact object. Most of the time that is either a white dwarf or a neutron star, depending on how much mass the core has. The shell is usually thrown out, especially during a neutron star collapses, where said shell smashes onto the neutron star and gets reflected back into space (that process is pretty complicated in actuallity, but that's the basic idea). Depending on whether some of the shell sticks around, the neutron star might accrete that mass and collapse further down to a black hole. The mass a neutron star needs for that is actually fairly well understood, and it seems to be a rather sharp transitional point, which is why we expect to find neutron stars of masses ranging very close to that boundary, but still below, as well as black holes that JUST made it over the boundary. That boundary, called the Tolman-Oppenheimer-Volkoff limit lies at roughly 2.3 solar masses.

This is, by the way, the reason why this observation is pretty cool: The team found a black hole that BARELY got over that boundary!

Edit: Type - Wrote 3.2 solar masses for the limit instead of 2.3. It should also be noted that the exact theoretical boundary depends on how exactly a neutron star is structured, or, in more technical terms, what its equation of state is, and we simply aren't sure about that at this point. But 2.3 solar masses seems to be in line with current observations.

Aside from the fact that microscopic black holes like that aren't actually dangerous.

it is common for black holes to be described as some cosmic vacuum cleaner that sucks up everythnig in its way, and so some seem to think a microscopic black hole would simply suck up Earth. But that's not how it works.

The gravitational force is dependent on the mass of the object and distance to it. Black holes like we're usually discussing have a really large mass, hence they exert a strong force of gravity. The really exceptional stuff only happens if you get very, very close to a black hole - because it focuses that much mass on a very small area of space (aka it's very dense). That makes the difference. With lower density objects, at this point part of the object would drag you to the sides, as it contains part of its mass, which means part of its gravitational force cancelled itselfs, limiting the amount of attraction you feel toward the center of the object. The singularity however has no outer parts that could cancel out some of this force. Hence you still feel a growing attractive force the entire time you approach the black hole, and at some point the force is so strong that not even light can escape.

However, for a microscopic black hole, it's difficult to get close enough for it to matter - other forces will counteract its pull of gravity. So what is more likely to happen is that the black hole would simply fall through the ground and fly towards earth's center, probably unable to capture much new mass along the route. Not exactly sure if it would be caught in some repeating movement cycle, flying up and down through Earth or something like that.

The black hole hypothetically created by the collider (or radiation hitting the atmosphere) would derive its effective mass from the energy of the collisions that created it - which isn't very much overall (a lot for a single particle, but not a lot compared to something like Earth. Or a chair.)

And if really fails to capture any mass or energy, it could even be that the black hole evaporates from hawking radiation eventually (I think very quickly, actually).

So we could in theory find something right on the border between a black hole and a neutron star?

Where light is almost blocked from escaping but not quite.

Would this display a you unique signature where high energy photons escaped, but low energy did not?

I’m a noob, but that seems like something we could search for.