Well, in the first place, there is nothing we can do if the Blackrock OP buys a lot of Bitcoin. The only thing that is worrying is that if they get the majority supply of Bitcoin, we will immediately see that they have the capability to really control it. the market.
If this were to happen, it is worth noting that it is not only blackrock that would be considered a whale in the market. Many organizations, sometimes even just individuals, who own a tremendous amount of btc and any movement from them can cause a shift in price. Your btc is still your btc and they can't really do anything to directly control it. They should be conscious of how their actions may affect the market and investors should be able to identify if it is only a whale moving or it is genuinely a reflection of how people perceive bitcoin.