Naa, they are almost certainly buying bitcoin.  For one thing a lot of this sideways price action is likely due to many traders who bought in at much lower levels and have been taking profit.  When you have large positions in an illiquid market it takes time to get in and out.  So the price action is totally normal even if it doesn't seem that way to you.

Also imagine what happens to blackrock when all those portfolios that bought bitcoin start to take profit and rebalance their portfolios when bitcoin rips to 100k, 250k, 500k.  Lots of ETF holders are going to be in balancing portfolios and that will mean selling bitcoin when it goes up a lot.

Blackrock is very smart, they are not going to rehypothecate the most bullish asset in the history of civilization 15 years into it's life.  That would be a good way to bankrupt yourself and then no more blackrock.

Just be patient, everything is working as intended, and as expected.