First of all, you can imagine any scenario you want. I can imagine a zombie apocalypse but that doesn't change the likelihood of it actually happening.
But let's assume for a moment your scenario came true. In that scenario, a hard fork created by BlackRock, whatever amount of BTC you hold on one chain you'll also hold on the other chain.
In other words, if you're holding 1 BTC you'll now also hold 1 BLKBTC. And somehow they've magically manipulated the price up of BLKBTC to $200k and the price down of BTC to $500.
What is a rational market actor incentivized to do?
  1. Are they going to sell $500 worth of BTC and put it into BLKBTC? This trade makes no sense for most people.
  2. Will they sell their BLKBTC and buy more BTC? This trade makes a lot of sense to current ideological Bitcoiners. Some people would definitely do that, myself included.
  3. Most people will do something in between these two extremes.
Again, you can imagine any number of variations of these scenarios but figuring out how they'd actually play out in reality is very different.