How do I repair a drywall ceiling when there is blown in insulation above?
Drywall is easy to patch without having to cut it back to expose a joist.  Here's how:

Cut a piece of thin wood to a length that is a couple inches longer than your hole.  A paint paddle or the thick side of a wooden shim works well for this, but any scrap piece of wood will do.  

Drill a screw partially into the middle of your scrap piece of wood.  

Using that screw as a handle, insert the piece of wood into the hole and center it so that it makes a bridge across the hole from the inside of your ceiling.  

Again using that screw as a handle, drive two additional screws through the existing drywall and into the part of the scrap wood that is hidden behind the drywall on either side of the hole.  

Remove the screw that you've been using for a handle.  

Cut a piece of scrap drywall into roughly the size and shape of the hole, and screw it into the piece of scrap wood.  

Use joint compound or Spackle to finish the cracks and screw holes.