Not an expert here, but as a cancer survivor I've had lots of meetings with doctors, oncologists, geneticists, etc. I've been told that cancer cells do turn black. I think it depends on the age of the cell though. Every cell in your body basically has a "kill switch" that causes the cell to die under various circumstances. When cancer occurs, it's because the gene for that kill switch is damaged, so basically the cell cannot die and just keeps dividing. After time, that cancer cell will continue to grow and eventually turn black.

I had colon cancer that spread to my lymph nodes. When my lymph nodes were removed, the cancerous ones did look black, according to my surgeon.