After all, the value of Bitcoin as an asset lies precisely in the decentralization of its supply.
Sure. BlackRock only holds the Bitcoin stock!. they don't like MicroStrategy company and Saylor who really bought the bitcoin on the market.
so whatever BlackRock bought, that does not affect anything on bitcoin supply. The supply is still intact in the wallet that owns the real Bitcoin. Because the company still thinks, this instrument is the same as their asset like a stock which I think is really different from others.