It may be that BlackRock hasn't publicly provided its address(es), but other Bitcoin ETF providers have (like Bitwose, and Grayscale I think), and chain analysis has seemed to identify the addresses. Im not very familiar with this specific organization, it be be likely they are known.
https://cointelegraph.com/news/spot-bitcoin-etfs-address-identified-transparency

But this is an interesting theory that Spot Bitcoin ETF providers on the whole may be doing reactional reserve. Meaning that they will take money for an ETF, backed by X Bitcoin, but then they don't buy more Bitcoin on the market when they don't take on those additional investors?