The table mdl_block_instance holds information for the blocks displayed for a course where pageid=courseid. If you look at the table you'll probably get a better understanding of how it works than if I explain it but to reset blocks for a course just remove the entries for that course from the ml_block_instance table.
Using
DELETE FROM `mdl_block_instance` WHERE `pageid` = n;
worked fine for both a single courseid
or a range of courseids.
I've got the default blocks set using site wide sticky blocks (using moodle 1.82+) so once i removed the entried in mdl_blocks_instance my preferred blocks were present (without any duplicates etc).
