you are wrong, blackrock is not that valuable, they are only ranked 118th most valuable company by market cap. the $9.42 trillion you mean is probably assets under their management, in management it means they manage these assets but do not have full rights to them.

while bitcoin's market capitalization is around 864.23b and is predicted to increase, and that is many times compared to blackrock's valuation. so for them to be able to manipulate the bitcoin market would be very difficult because they would need to mobilize a lot of resources just to move the market which could actually harm their company.