Until recently Circle were parking their cash in uninsured deposits [1]. Moving to repos may be about the debt ceiling, but it's more likely that someone hired a treasurer.
Circle didn't always use BlackRock. The "someone" is Circle. Maintaining a mix of Treasuries and repos is standard treasury management. I don't see a link between this rebalancing and the debt limit fight.
> blackrock thinks a usa sovereign default is a significant risk
It's a manageable risk, so it's managed. That's risk management. (Repos also yield more than Treasuries. Again, this probably has nothing to do with the debt limit.)
[1] https://www.ft.com/content/7c9b2234-c298-4508-b59a-fce49f6bc...