[joshuak]

Looking at the wikipedia article I can see where one might be confused.

    A blockchain is a growing list of records,
    called blocks, that are linked together using 
    cryptography. Each block contains a cryptographic
    hash of the previous block, a timestamp, and
    transaction data (generally represented as a
    Merkle tree).

The Merkle tree referenced here is with respect to the organization of the transaction data contained within a block, not the blockchain itself.

Merkle trees are used in various ways within cryptography in general and cryptocurrencies specifically, but blockchains and Merkle trees are distinct data structures with different uses. The colloquial use of "blockchain" has perhaps made the word somewhat ambiguous in some contexts but not in the context of cryptographic data structures, and Merkle trees are in fact formally defined.