How does blockmix in scrypt work? What is the meaning of the H function in scrypt?
I'll sum up the comments: according to the original paper, $H$ is any hash function which, ideally, uses limited parallelism$^1$, with Salsa20/8 being a candidate.

Footnote 1: In order to make the resulting memory-hard function less susceptible to attacks (which make an essential use of parallelism).