The colour sequence of the Great Work is older than its Latin names, and tracing it back changes what the stages look like.
The Greek names
The Graeco-Egyptian alchemical writings, the earliest substantial body of alchemical text that survives, describe the work as a progression of colours: melanosis, the blackening, then leukosis, xanthosis and iosis. Nigredo is the Latin translation of the first of these, and the Latin sequence that this atlas records is a rendering of the Greek one.
What the blackening was on the bench
In the Greek material the colours are colours of a metal or an alloy being worked, and blackening is what a prepared metal surface does under certain treatments. The stages describe an observable sequence in a vessel before they describe anything else. Later writers read the sequence spiritually and later writers still read it psychologically, and each of those readings is real, but the observational layer is the oldest.
Why the first stage carries the weight it does
Of the four, the blackening has attracted by far the most commentary, in alchemy itself and in every later system that borrowed the scheme. A sequence whose first step is dissolution and decay is unusual among models of improvement, and that is very largely why the vocabulary has travelled so far outside alchemy.