Purple discoloration at stem joints. Why?

[WVTomatoMan]

I've had and have seen purple to almost black discoloration of pepper plant stems at the joint of a leaf branch as well as running vertically up the stem. This occurs on otherwise healthy plants. At first I thought it was a magnesium deficiency caused by cooler weather. But then I've seen it last an entire season. What is it? What causes it?

Thanks.

Randy