The only thing that keeps a blockchain appearing to be a rebase-only straight line and not an otherwise ordinary merkle tree is the consensus algorithm on what the "current state / last block of the chain" is.

The difference between a blockchain and a merkle tree in no way resembles that between a B tree or LSM tree. It's closer to the difference between a Binary Search Tree and a Red/Black Tree. They are both Binary Search trees and most operations are the exact same, but Red/Black Trees have an extra "color" attribute and rely on that for a better balancing algorithm than the "default" BST algorithm.