You weren't entirely wrong. Black rock originally started as part of black stone. It split in 1995 to become independent. However, I would see that as a strategy for plausible deniability despite linger business connections. Something that often occurs to avoid monopoly accusations.
Also interesting that both of them are so similar to the black cube symbol of Saturn, which is a cosmic force that enforces structure and order but also confines individuals within the bounds of time, space, and material existence.
Source that isn't paywalled: https://www.cnbc.com/2017/06/22/blackstone-or-blackrock-schwarzman-and-fink-did-it-on-purpose.html#:~:text=Larry%20Fink's%20BlackRock%20started%20as,on%20his%20own%2C%20Schwarzman%20recalls.