2 years post-op. My nipples are dark purple to black and tingle like pins and needles. Advice?

Generally dramatic color changes such as purple, black, or white, and numbness or tingling indicate various possible issues of blood supply being compromised. Other than being uncomfortable and looking unusual, this is a big deal because it indicates that your nipples/areolas might be at significant risk.
This could be due to thin tissues, which thin further over time, combined with implants that are too large and/or too projected, and more so if multiple cuts have been made in the past around the areola. Smoking can further compromise blood supply. 
You may need your implants removed completely or changed out to smaller or less projected. I definitely agree with the comment that if you are a nicotine user, you should stop right away since in cases of necrosis the nipples can turn black, die, and fall off.
Hypercoagulable conditions, due to certain medical condition or medications, combined with the circumstances above may also be a factor.
Without having close knowledge of details of your case and medical history, my only recommendation is that you see a board-certified plastic surgeon familiar with breast revision surgery quickly.

I would suggest finding another board certified plastic surgeon to check you. Also, when your nipples turn dark, please take photos to show the surgeon. If you smoke or use any tobacco products, please stop now.

Thank you for your questions.  I would recommend seeking attention from a Board Certified Plastic Surgeon to evaluate you in person.  Without seeing you in person it's difficult to determine what's possibly going on.  Best of luck to you.