It might work something like this.
Blackrock has 2,200 Bitcoins.
Blackrock lends 90% of them or 1,980 Bitcoins to Grayrock. The resulting supply is now 2,200 + 1,980 == 4,180 Bitcoins.
Grayrock lends 90% of their 1,980 Bitcoins or 1,782 Bitcoins to Palerock. The resulting supply is now 4,180 +...