If BlackRock wanted to increase the supply, for example,to 50 million bitcoins,which is needed to develop the Bitcoin software.
In simple terms,they would need to change the Bitcoin Core software code to allow for 50 million coins and upload this new version of Bitcoin to the network.
This would create the original Bitcoin chain and the new version.To download the new chain,all node operators,miners,users developers,and exchanges would need the updated version.
However,Bitcoin users,miners,node operators (who verify new blocks), exchanges,and miner developers would not support such a change but it would certainly devalue their coins.
This is not a new scenario. Bitcoin Satoshi Vision (BSV) and Bitcoin Cash (BCH) are examples of companies that would certainly.Similarly,any new version of the Bitcoin network that attempts to change the cap to 21 million would also fail.
Bitcoin is the most dynamic,flexible, seamless,and transparent network ever created by humans.
While there is no absolute guarantee that Bitcoin will remain at its 21 million limit (because no one has the authority to enforce it),that is precisely the beauty of Bitcoin.