In alchemical texts the use of crows or ravens to represent nigredo or death is common, and it is often the appearance of nigredo that would signal the beginning of the opus or alchemical work.
The Ten of Air from the Vision Quest Tarot is an example of a nigredo state. Buzzards, vultures and such birds of prey are often used to symbolize negative or invasive thoughts that need to be overcome, In this case, the scene can be associated with "mortificatio," death and "putrefactio."
Thoughts give form to the world around us, and this is the reason why we must be aware of how negative thinking can affect our manifestation of reality. Just like Key 13, Death, or Key 15, the Devil could represent an aspect of nigredo, so could any situation that brings a sense of darkness to our soul, or a sense of despair. We should look carefully at the cards in a spread that could signal a mental state of helplessness, desperation, inability to take action, cards that bring forth a sense of remorse, shame, pain, defeat, deep suffering, soul searching or complete disorientation. The element of Air is usually assigned to swords in tarot. This is the element that rules the mind and our thoughts, but it is also the element of Spirit and Ruach.
The IX of Swords above is often associated with the archetypal "Dark Night of the Soul." But remember that nine is also the number of the Hermit, and can signal wisdom achieved after trial or sacrifice. Take a closer look at this IX of Swords; look at the decorations on the bed and the sigils on the quilt; look at the way that the swords are lined up against the black background. We know that this is a card that requires deeper understanding because this individual is going through some form of personal transmutation.
The cards below could be interpreted as different ways to experience nigredo in our life. These examples are from the Waite Smith deck, the Sola Busca 3 of Swords, the Fey Tarot and Meier's Atalanta Fugiens (M.P. Hall collection)