I can't see long blocks once a week working for math, music (other than appreciation), or foreign language.
I could see it working for other subjects, especially if you have a student who gets immersed in a subject and doesn't want to change away. I could see it being especially beneficial for science, history, literature.
A suggestion: test-drive it by slightly lengthening all but one subject on 4 days a week. Reserve one day for the subject you have the best ideas for what you'd do with a long, uninterrupted day (science/history field trips, shakespeare plays, whatever) and block schedule that one, removing it entirely from the other 4 days.
I could see math/language working okay on an every-other-day schedule (I wouldn't go less than MWF), which would free up two days for long subjects if your trial was successful. If you did that, I'd go with morning/afternoon blocks on both to lessen brain shutdown.