Who created the Bitcoin blockchain? Satoshi Nakamoto wrote the theory about Bitcoin in his paper, but did he (or they) also launch the Bitcoin blockchain?

Like: Peter Higgs founded the Higgs particle in theory, but he didn't find the actual particle...
Satoshi did start the blockchain.  The first block, known as the genesis block, is embedded in the source code of the reference bitcoin client.  The rest of the blockchain is built on top of this block.

The genesis block contains the headline "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks" to prove that the genesis block was created on or after Jan 3, 2009, and that Satoshi was not working on mining a longer blockchain in secret prior to this date.