I have an uninsulated cinder block house in zone 7. I was going to re-do the attic insulation with 2" of spray foam then cellulose. From the top plate you can access the cinderblock cavities about half way down the wall. The insulation contractor wants to fill them with cellulose is that a product that would work for this application seems like a bad idea to me. When I did a search it seems in the old days they used to use vermiculite or perlite for this application here back in the 1950s when they house was built

1950's house aren't usually super strong...Cellulose is 4 times the weight of Fiberglass. Don't know how much you are putting in but you might wan to calculate it out.

For the walls it is much better to put a layer of foam on the outside like under the siding. I don't think adding to the block cavity is going to do anything for the cost. Much like insulating a steel stud wall does nothing.