I don't think the name "blockchain" is trademarked, but that's a good question. The Bitcoin community made up the word blockchain. Everyone was using it to refer to the public ledger that starts with the Genesis block, and which miners add new blocks to. It was from that name that Blockchain.info started collecting the public data on the blockchain, like new transactions, and putting it in easy to view format. When trademarking a name the name must not be a word commonly used in public, and even then the trademark holder has to defend the name if people start to use it that way. For example, that's why Xerox would prefer people say "copy it" than "xerox it", because the latter weakens their trademark.