I wrote about this
here:
Blockchain or Block Chain?Bitcoiners spell Bitcoin’s transaction database two different ways, and it is likely that you will see both used: the blockchain (one word) or the block chain (two words). When discussing Bitcoin’s transaction database, they both mean the same thing. There isn’t an official term because there is no official Bitcoin dictionary. However, Satoshi Nakamoto, the creator of Bitcoin, called it the block chain on multiple occasions. Also, the most popular Bitcoin wiki, bitcoin.it, calls it the block chain. For these reasons, block chain is considered to be more correct, however blockchain is often used colloquially.