8 days post op arm lift. Is it normal to have my skin black by the stitches? (Photo)

A combination of things are happening at the incision site.  Some of the discoloration is superficial tissue changes that will improve with time. Sometimes the discoloration is from poor circulation to the skin at the incision. This could be secondary to the tension placed on the closure.  The best thing to do right now is just allow it to heal and keep the area clean.  Make sure your surgeon is evaluating the incision often to make sure that it does not get infected. Worst-case scenario you might be looking at a office procedure to revise  the scar down the road. 

I am sorry that you are going through this inconvenience with your surgery.Answering your question:the changes in your skin might be due to blood flow restriction that might be secondary to the tension of removing the excess skin.I notice that there might be some redness associated to the skin changes.Make sure to keep your surgeon informed of this changes so that he can guide you in the best way to prevent major complications.best wishes.