There isn't a central group that controls bitcoin. tldr: Blackrock can't control bitcoin.
Let's say that Blackrock wants to change Bitcoin, which means changing the developer code. They would effectively need to convince a majority of miners and users to accept their new code that they want to apply to the bitcoin network. Historically this has not been a group that reaches consensus easily on a change.
Alternatively, Blackrock, or anyone, always has the option of copying bitcoin (& its corresponding owners) to a new, separate version of Bitcoin with whatever change Blackrock wanted. This has occurred in the past with other groups and pretty much fizzled out.
IF Blackrock wanted to control/change bitcoin, buying out the miners might be the best option but there is limited value in that. As they approached control, it would scare off bitcoin owners and the price would plummet. To control bitcoin is to kill it.
Re: Blackrock's current stake. They're reporting another 10K bitcoin owned yesterday, up to 150K. At this growth rate, Blackrock should surpass Microstrategy in the upcoming weeks. Other groups still have more.