Black phosphorus (BP) is a layered semiconductor with outstanding properties, making it a promising candidate for optoelectronic and other applications. BP synthesis is an intriguing task largely due to the insufficient understanding of the synthesis mechanism. In this work, we use density functional theory calculations to examine BP and its precursor red phosphorus as they are formed from P4 building blocks. Our results suggest that, without external effects such as pressure or addition of a catalyst, the precursor is energetically favored in the initial steps of the synthesis, even though BP is the more stable allotrope. The higher energy of BP is dictated by its 2D geometry that gives rise to the higher number of high-energy strained bonds at the edge compared to the 1D geometry of red phosphorus. The elucidated BP formation pathway provides a natural explanation for the effectiveness of the recently discovered Sn/I catalyst used in BP synthesis.