As long as they will keep buying not selling don't see problems
That's actually the problems!
If Blackrock keep buying Bitcoin, it means Blackrock will own all Bitcoins except there's person don't want to sell his coins at any price. After that, Blackrock can do anything with the coins, they also can force people to only buy their fake Bitcoin i.e. ETFs and forbid people to withdraw their coins to exchange or wallet.
It makes everyone who want to buy and own Bitcoin must buy ETFs instead of the real Bitcoin.