Darkness of the heart refers to the distortion of our desires, the degradation of our consciousness that happens because of our distorted desires, thereby making us succumb to our lower indulgences.

The Bhagavad-gita begins with a statement that signifies darkness of the heart: he is nepotistically concerned only about his sons and is eager for the destruction of his nephews – all because of his attachment to his sons.

When we are overcome by our lower desires, then we too are unable to perceive the need for unbiased vision. We are unable to see that our biases are distorting our vision, either of people whom we may demonize based on our pre-conceptions or of objects which we may crave for based on our conditions. Thus, we may succumb to immoral indulgences because the darkness within the form of impurities makes the impure, the immoral, the reproachable appear to be irresistible.

Just as when we encounter darkness without we immediately try to get light, similarly we need to turn to scripture. Then whenever our perception starts running contrary to scripture and start craving for worldly gratification which is actually a source of tribulation while appearing as a source of pleasure. Then our intelligence needs to trigger into activation and help us understand that we are being enveloped by inner darkness, and then we can immediately turn to the Gita’s wisdom. And we can illumine our inner world and thus protect ourselves from the perversion of our vision, and the degradation thereof, and gradually as we study and apply the Gita. Krishna’s presence will become more and more prominent and consistent in our consciousness, and thus our inner world will become illumined by his guidance.

Thereby we will progress steadily towards the life of morality and spirituality, attaining supreme satisfaction through absorption in Krishna.