No one can say for sure, as long as Blackrock does not confirm. That's the beauty of Bitcoin, it's pseudonym: The transactions ledger is public, but the adresses are anonymous as long as the can't be traced back to a person or institution.

One thing is for sure, in order to launch their Bitcoin ETF, they need to stack massively somewhere.

I think they have an agreement with Coinbase as their custodial wallet provider.