If a significant amount of Bitcoin is held by individuals and they choose to sell Blackrock fork then BR fork price goes down. It’s as simple as that. Like every other asset, good, etc. A mass of people selling something because they don’t want it makes the price go down. Think about Europe selling (or even not buying) silver in the late 19th century in favor of gold, silver price went way down relative to gold even though Asia bought that silver. There needs to be reservation demand, market making alone can’t sustain a certain price. This stuff about arbitrage doesn’t mean anything, nothing unique to Bitcoin or forks.
Discussion
If the price of a thing goes down, it is because people are not willing to pay for the thing -- not because people are offering it for sale.
> a mass of people selling something because they don't want it makes the price go down.
What are they selling it for? If people in Asia traded silk or opium for silver, did that make the price of silk and opium go down when they sold it?
If not, why isn't their act of not wanting silk or opium equal to European's act of not wanting silver?
If it did make the price of silk or opium go down, why did Asian selling gold (which Europeans were buyinh) not make the price of gold go down?
Finally, if Europeans traded silver directly for gold from Asians, why wasn't the Asian selling of gold acting on price exactly the same way as the European selling of silver?
You’re talking yourself into circles. I can’t help you any further. Read more Austrian economics.