Sorry I have no idea what is really is. But if it's helps, my Brunfelsia 'Isola' was originally green then when moved to full sun, it developed that same purple pigment all over even along the stems. It doesn't seem to be a bad thing.

Yeah, I second the "brunfelsia turn all sorts of colors" sentiment. They're super easy once established but if it's brand new to you I can understand that a single spot can be stressful. It doesn't look malignant to me. Plus, you've got a flower bud.

This is good news. Thanks for the tips and encouragement.