I'm fairly sure they announced it on this list some time back.
For me, it solves the issue of wanting to use the command-line for mail in
general but also being able to access mail from a mobile phone (or HTML capable
mail client) using Maildir backed dovecot and IMAP.
The basic approach of having what would be a sequence in MH implemented as a
file containing a list of message files works well. Being able to get a
threaded list of messages is really useful, and fast. Integration with other
tools like notmuch for searching is easier than for nmh.
I still use nmh for viewing and composing messages via wrapper shell functions,
however. mblaze lacks the flexibility of mh-format and the hard coded defaults
weren't to my taste. But it is a very capable tool.