Typically, they include a cryptographic hash of the previous block, which I would think would imply singly linked. (Since otherwise, you need to predict the out

Typically, they include a cryptographic hash of the previous block, which I would think would imply singly linked. (Since otherwise, you need to predict the output of a cryptographic hash function, which is by definition not possible.) And the big ones (e.g., Bitcoin) are more properly trees, though mostly all branches except one will be ignored.