This is a question that hadn't occurred to me till you said that: Where will Blackrock buy their bitcoin?

They aren't going to set up an exchange presumably, so it's like, just buying it on Coinbase? Or will they spread it around the other markets too?

If I'm selling on Coinbase then presumably I can't say "Only not if it's Blackrock what wants to buy", so that'd mean you're saying don't sell on Coinbase at all?