Could dark green / black nipple discharge be caused by my 21 year old saline implants? (Photo)

This needs to be evaluated for sure.  Evaluation might include physical exam, mammogram, MRI and surgery. Your silicone implants need to be removed and replaced at the very least, with en bloc capsulectomy.

You need to be evaluated by a breast surgeon to see what this fluid is. Getting cytology and cultures may be a good way to start.

There are a number of "could be" possibilities, but since this is definitely abnormal You should have it fully evaluated. I have seen a similar issue only once in 25 years and this fluid also exsisted around the implant within here pocket yet despite extensive testing (common and rare cultures, cytologic exam, chemical exam) no "cause" was ever determined. She was never ill nor symptomatic fro  it.

Maybe But this is very concerning!! Seek ONLY IN PERSON opinions ASAP................................

Based on the information you provided (normal mammogram, assuming no visible or palpable mass, no pain or tenderness, no erythema, saline implants), infection is unlikely. Leaking implants would deflate and the saline would be absorbed. Discharge from your nipple is most likely to be old remnants of lactation, but a cytology evaluation would be recommended to rule out a ductal carcinoma, especially if the discharge becomes bloody. Please see a breast surgeon or ob-gyn for evaluation.