[shrewdcomputer]

> The black hole is actually quite special. It is different from all other known black holes, and its existence is difficult to explain with standard binary evolution models.

Given this and the difficulty in spotting dormant black holes, it will be interesting to see if these black holes are suprisingly common. Similar to the way gravitational microlensing led astronomers to find far more low-mass planets than they expected.

[twawaaay]

If you are looking if something is "surprising" then it is not surprising, by very definition.

We know BHs should be pretty common. Earlier generations of stars were composed of larger and more short lived stars (due to their low metallicity) that should mostly end up as black holes.

As to detecting gravitational microlensing it is not as easy as it sounds. A black hole remnant of a star will be microlensing the light just like a star -- only without very visible reason for the microlensing to be happening.

In practice what you would see is when a black hole moves in front of a star that star's light is suddenly magnified without any good reason. It is one time event, it is rare and afterwards there is nothing else to study. These types of events tend to be very hard to capture because they require you to look at entire sky all the time.