There are a few things going on I think:
Black Rock is the big giant owning everything and packaging them into investment assets for your retirement funds and sovereign funds, they are the house, they don't speculate like private equity funds, they don't guarantee things appreciate in value or "create values" above market return. They just make a cut in management fees.
Black Stone is sort of a private equity fund that leverage, buy, harvest, sell, and let the asset collapse at the new ownership.
I can see Black Rock, Vanguard, Fidelity owning everything in the world eventually, but Black Stone? They are hit and run and are not a buy and hold kind of company attempting to get above market return.