AgentSmith:

"PJF - your first pic; "and there are no walls to provide any bounced light" - the ground around the block would bounce light back and forth from the back of the block. The center of your shadow would be dark enough for the untrained human eye to call black."

Fair point (though you overstate the case for bounced light from the ground in this instance), and it's frustrating that my little set failed to illustrate my point. I'll adapt it to make it unambiguous.