What is a Huffman tree for a Block-Code?
In your context, a block code of length $\ell$ encodes $\ell$ letters at a time. The domain thus consists of all words over the alphabet of length $\ell$. Unless you're told otherwise, the assumption is that the individual letters are independent - the probability of a word $\sigma_1 \dots \sigma_\ell$ is the product of the probabilities of the individual letters.