I didn’t try Pascal’s suggestion, but AFAIK, if you don’t want to go one-by-one (especially hard if your block instances are non-uniformly scaled so BlockEdit won’t let you…), this can be done only via script. Luckily, your RhinoScript works now , so try this:
DIG_BlockObjectsLayers.rvb (4.4 KB)
It works one more complex and nested blocks as well; plus you can either pick block instances or select from the list…
EDIT: Please note, it will only change the layers of objects INSIDE blocks, not the block instances themselves. The instances will stay on their original layers but these are easy to move via Rhino UI.