My school is on blocks and uses the UCSMP books. We have been on blocks for three years and used the books for two. I frequently get notes from substitutes saying I left good plans - I was out a lot this year for professional conferences and the like - so I guess I am doing something right in my plans anyway.
I don't find leaving plans for blocks any different than leaving plans for 50 minute periods. I NEVER leave my grade/attendance book at school, so at the beginning of the year I make up a roll sheet for even days and odd days. I attach this to my plans. I have a standard page that details the "where to find stuff" information and, as the year progresses, I leave notes as to which students can be especially helpful.
I leave detailed solutions on transparencies for homework problems and then instructions for class work and homework assignments. Since I never know how comfortable my substitute will be with the material, I always simply expect them to babysit. Those who can help the students do so and those who do not feel that they can be helpful in the problem solving are not in an uncomfortable situation.
If I do not have warning that I will be out - for example waking up sick - I do have the emergency instructions in the office. I find the fax capability of my computer invaluable!
Hope some of this is useful! One last note, leave approximate time frames so that your sub will know about how much time to expect to spend on each activity and leave more than enough activities to fill the time. I also immediately discipline any students about whom the sub complains. I usually find that the only time any sub has any discipline problems is the first time I am out during the year. Word travels fast!