Concise answer
Pigment can darken when treatment irritates or inflames the skin, when sun exposure continues or when the original pigment pattern was not correctly identified. Darker skin types can be especially prone to post-inflammatory change.
Layering strong acids, scrubs and procedures can damage the barrier and trigger more colour even when each product is marketed for brightening.
The first step is often to settle inflammation and reassess rather than escalating intensity.
When evaluation may help
Stop unreviewed strong products and seek care for burning, blistering, swelling or rapidly spreading darkening after a procedure.