What is the meaning of Blackening?

present participle and gerund of blacken

The act or process of turning (becoming) black in colour.

The result or an instance of such an act or process; a black stain or mark.

Any of various (dark-colored) surface treatments for metal substrates, to inhibit corrosion; on steel, generally, synonym of bluing.

The act of defaming or sullying.

Source: wiktionary.org