I don't really know a whole lot about the graphics class (I've used it a few times) but I don't believe you can call ANYTHING on a null object.
blockGraphics = null;
blockGraphics.clear();
is the same as calling:
null.clear();
Which is going to give you an error. Typically you'll want to take a movieclip or other such object and get it's graphics instance:
blockGraphics = mc.graphics;
blockGraphics.clear();
blockGraphics.beginFill(0xFF0000);
blockGraphics.drawRect(10,10,10,10);
would draw a red rectangle inside the "mc" movieclip.