How many black holes does the Moon hide?

The Moon has reached first quarter in its moonthly orbit around the Earth (a small mis-spelling of the word month, and you realise where the word originates from!). You have about 6 hours to admire the moon after sunset, until it too sinks below the western horizon, due to the relentless rotation of the Earth.
Most of us are used to the familiar sight of the first-quarter Moon through our own eyes. But the sight through an X-ray telescope is perplexing, to say the least.
As described in my previous post, X-rays originate from very hot gas. In the case of the Moon, the X-rays originate from hot gas bursting out of the Sun, which are then reflected off the Moon.
In 1990, the Rosat observatory took this image of the Moon.
The Moon, hiding many a black hole...
It is somewhat ironic that X-ray astronomy began in the early 1960s, with the aim of detecting X-rays from the Moon. The plan was to get some clues about the conditions on the lunar surface before Neil and Buzz got there before the decade was out.
Rosat
But it took another 30 years to detect those X-rays from the Moon!
Even more ironically, it turned out that the Moon wasn’t the really interesting bit after all. Look at the X-ray dark half of the Moon, and you notice that it is actually darker than the deep space behind it…
And that raises the question – where are all those X-rays behind the Moon coming from? (The dark side of the Moon is not completely black, as some charged particles from the Sun do collide with the far side of the Moon, producing X-rays.)
To know what was causing the X-ray background, we had to wait for Chandra, which showed that at least 80% of the X-rays originated from distant star-swallowing black holes – half of which are about the mass of the Sun, the other half being a billion times larger! (see here)
The image taken by Chandra is both boring and astounding at the same time! Boring, since the image is just a collection of dots. Astounding when you realise just what those dots actually are.
So how many black holes does the full Moon cover?
Well, it depends how sensitive your X-ray telescope is and for how long you look! But there will be anything between 100 and 1000, depending on where you look in the sky (it is easier to see black holes looking directly out of our galaxy).