The dark blob you see moving around is most likely the embryo. As the chick develops it will get bigger and more active.
The embryo sticks to the egg yolk. The egg yolk does have some movement in the shell, but is tethered to the membrane by the little yolk cords. Normally the embryo will float to the topside position after a bit (hence the thought that the development is only on one side...no, your yolk is not stuck to the shell unless you've lost all moisture in the egg which would result in a HUGE airspace) and while there's still room to do so. Do not worry about if the veins, etc., appear to be visible on only one side of the egg. If you see veins and/or a little blob, all is good. Do be worried if the whole egg is dark early on or if you have NO air space or a drastically shifting air space. Too, I find candling from the air space end of the egg gives best visual results with my powerful little candler (an electric stick type thing). Chicks positively dance ("Turn off that durn light! Eek!")
Keep turning the eggs. As the days go on (and if you've got a good candler) you'll see which eggs are viable and which are not. Sometimes the embryos are not visible until well into the second week, so don't throw out any questionable non-rotten eggs just yet. You'll get a real good idea at about Day 7-14. I candle at Day 7, Day 10, Day 14 and then finally at lockdown on Day 18 to make sure nobody stopped developing mid-way through.
Good luck!
