This means blackrock can determine which network they will consider the appropriate one so it is reasonable to think they might think about pushing another bitcoin hardfork and setting their own rules to consider it the appropriate network.
You are overthinking here, Blackrock investing in bitcoin is something that logically they should have done years ago because they are an investment company.
Blackrock is not a threat, even if there is a hardfork and they support it, there is no threat for those who use bitcoin because they can use original anytime. Does price of bitcoin forks or altcoins matter for you when you want to have a decentralized network and improved privacy? You use bitcoin because you believe in these fundamentals, right? That's all that matters for you then, not a cash flow.