Satoshi Nakamoto always referred to it as such:
block chain
However, I see the term "blockchain" used a lot anyways. Do you think we should use Satoshi's terminology to prevent confusion with Blockchain.info, who will usually just refer to themselves as Blockchain? Or am I worried about nothing?

I always thought the proper way was one word
blockchain but I wasn't aware that satoshi used it as two. I think blockchain.info was silly (or maybe genius) idea to name themselves after it though as it always leads to confusion or people mistakenly thinking they are the official block explorer.