Ok, just wanted to check that first.
Sometimes they do develop weird spots that are harmless, sometimes they are very harmful... I have personal experience determining the difference (Kaira had Yellow Fungus when she was young), but I'm not seeing anything like that from the picture. And you do mean purple, not black-ish? Black-ish could indicate necrosis (sp?) (dying tissue) but I'm thinking it's not anything serious.
If it does look more like the purplish side bars your beardie has, and not black-ish, I'd say keep an eye on it, but wait for another opinion, I'm not one of the experts on the site.
Sorry I couldn't be of more help, others will know more.
Abi
*Edit* The shed on the tail...that may mean the leg is about to go into shed too, that may be what this is...still, wait for another opinion.