Shiva as Mahakala represents the void at the dissolution of the universe. Time and space are contained in Him. Thus Mahakala is time. Here is a look at the deep symbolism of Mahakala.
Mahakala form of Shiva is typically visualized as black in color. Everything is absorbed and dissolved in black. All forms merge into Mahakala. His all encompassing nature is unimaginable.
All the filth on earth born out of human ignorance is dissolved by Mahakala. He burns all human creations into ashes. He transforms everything back to its original state.
Black of Mahakala thus represents total stillness – the original state of the universe.
Mahakala is that bindu (the black dot) into which all creation returns.
Nasadiya Sukta in Rig Veda explains about this bindu or black dot –
At first there was only darkness wrapped in darkness.
All this was only unillumined cosmic water.
That One which came to be, enclosed in nothing,
arose at last, born of the power of heat.
When human arrogance and ignorance brims over and the foul smell starts to contaminate the universe, Shiva rises up as Mahakala. Just like he did to infamous Daksha Yagna, he reabsorbs everything by converting them into white ash.