Leadsled, This is pprune, the center of the pedantic universe. I could have told the OP the regulation regarding block altitude clearances is the same as for a fixed altitude. You have to abide by it. A block altitude clearance is normally at the request of the flight crew due to weather or maneuvering. A cruise clearance is issued by ATC in lieu of a fixed altitude. I look at them as two totally different things.