I wouldn't worry about that too much. First of all, Blackrock does not hold any bitcoins, they use custodian, Coinbase. So it's coinbase that would end up holding all the coins from the new fork. And I don't suspect the distribution of forked coins would be covered in the contract between Blackrock and Coinbase.
Do you think Blackrock was stupid enough not to add this clause to the coin held by Coinbase?
They have an army of lawyers and not one of them thought of that?
Besides, I expect they would pull them out to their own custodian if they would try such a move!
Blackrock would need mining power to fork bitcoin and maintain it. Otherwise it would just be a dead fork...
Blackrock is the first and second largest shareholder in nearly all listed companies in the US.
And more than that, it basically owns through debt Core Scientific, which had 28exa and just signed a deal for 15exa yearly delivery, and 15 in 2025, they've got basically 10% of the hashrate just through one alone!
Also, they have their own advisors in the Bitcoin Mining Council, that's a sign of how much power they have over those companies.
They won't do it, but theoretically, they have the power!