I have 2-D sketch that appears entirely black, but the minus sign after the sketch's name indicates that it's under-defined.  If all of the entities are black, how can I find what's causing the sketch to be not fully defined.  Everything was OK until I made a block.  The block didn't behave as I hoped, so I exploded it.  Only after that did the under-definition show up; before making the block the sketch was fully defined.