Dark spot in scar, what could it be?

It's difficult to say what a dark area along your incision could indicate without an exam or photos. It could be a dried blood, retained areola, or something else. Your surgeon should be able to guide you once the photos are reviewed.

Without an image it is difficult to conjecture with precision as to the cause of the color change. Recurrent bleeding is rare this far out, so the other possibilities are problematic wound healing, or infection. If you have redness and pain, infection is likely. If you can’t see your surgeon, go to urgent care to be evaluated for possible infection.

Thank you Janegrace for posting your question here,I will start by stating that my answer is based on the limited information provided, for a more bespoke plan please consult with your plastic surgeon.
I am sorry to hear that you had to go through multiple drainage procedures, though uncommon , hematomas can occur, I hope you feel better soon.
When it comes to dark spots within the nipple scar it can be a number of things and it is hard to assess without examining you or at least looking at pictures. Just by virtue of their location and assuming that they are small in size , if I had to guess it could be a suture granuloma, or a suture that is yet to be absorbed that is trying to make its way out , that being said , I cannot definitively advise you on the matter without an exam .
I hope this helps, best of luck!