Water Gardening: Dividing Lotus: share your tips and techniques, 1 by darlindeb

darlindeb wrote:
Sometimes they look dark or rotten and it's just a covering over a leaf that will emerge. If you have another portion of the plant that looks good, it will probably grow okay.

In this photo you can see how the back tip is dark. That is a pretty normal from my experience of dividing them.

You might want to take a picture of the tuber and e-mail it to whomever you purchased it from and ask the seller what they think.