There's more information about aura in general explanations of yoga and chakras. If you google "chakras and endocrine" - you'll get a fairly interesting picture of what the chakra system is. It corresponds to various glands and various glands correspond to various substances that are crucial to the proper functioning of the body. The thing about yoga and modern science is that science has identified these organs and has accurately explained what they do in the body.
Yoga has taught people how to maintain one's good health through stretches and forms of concentration which limber up the spine in order to manifest the excretion of these substances in moderate, healthy amounts. Also, the excretion of these substances, apart from being part and parcel to the whole operation of the body - what I am trying to say is that, it's one thing to stretch and let these substances come out as they should (modern living does not "encourage" - so to speak - the natural functioning of these endocrine systems and these glands and what not).
It's a whole other thing to become a balanced person that does not have to consciously make efforts to allow for the very natural and essential excretion of these substances. In other words, yoga and what not are important at categorically targeting all of these points that have been identified as crucial regions of the body, in terms of health, that is, and - by science.
What is an aura? These substances - like testosterone, estrogen, progesterone, insulin, thyroxine, lymphocytes of various kinds, melatonin and other regulatory hormone, etc - they are like anything else - they have colour. But they are also unlike anything else. They carry feelings unique only to biological systems. The pleasurable feeling of a healthy lower back, the pleasurable feeling of a sleepy brain seeped in melatonin, or a body seeped in the healing power of lymphocytes - etc - this is only transfigurable. Ie. It is the quintessence of being human - it is part and parcel to our macroscopic psychological opinions about our own sexuality, our own identity, our own emotional well being, mental well being, fitness (in terms of how we approach eating), and livelihood (in terms of how we approach working in society).
What is an aura? It's a general phrase that people use colloqually - "Oh! You have a very nice aura today!" or "Wow ~ what a bright complexion." Pregnant woman are glowing!!! Come on! It's a documented social fact. And why are pregnant women glowing? Because they're brimming with hormones as their bodies give health and sustenance to a growing life form.
So that is what an aura is. An aura is a transfigural dimension in which somaticly defined substances (which we have come to understand scientifically as products of glands in the body) - where these substances become a flowing part of the body - giving health and proper functioning to our organs, as well as something of a luminescent glow - barely noticeable to the eye - but obvious to the cognizant part of our intuition - and in exceptional cases (say - in the case of a pregnant woman - or a very, very happy person) - in exceptional cases - aura is noticeable by all people.
Long story short - if your endocrine system is blocked for one reason or another - the associated aura for the substance released by that gland will not come about - and the reasons are fairly simple to understand.
Hope that helps.