At day 13, depending on how bright your flash light is and how you are candling, all you will see is dark blob and air cell at one end. Seeing that is a sign that you've got developing chicks, which you probably won't see move unless you stand very, very still and hold the egg for awhile.
Take a regular egg out of the fridge and candle that. You will see the difference. If not...if you are seeing transparent type egg with light shadow yolk moving, then no, you've got nothing.
And yes, blue shells, olive shells, and dark brown shells are very hard to see anything. All you will see is dark blob and air cell with no distinction....if that. Sometimes the shells are do dark to see anything. I usually can see veins on lighter eggs, but not always.
LofMc