Ayyappa devotees wear black dresses to symbolize detachment from worldly desires and to enter a state of piety and discipline during the deeksha period. The black color is also linked to a historical story where Lord Ayyappa assured Lord Shani that his devotees would wear black, a color associated with Shani, to appease the deity and seek his blessings. 

• Detachment from materialism: 

  Wearing black signifies a renunciation of material things and worldly perceptions, helping the devotee focus on Ayyappa. 

• Discipline and piety: 

  The black dress is a visual representation of the strict vows and disciplined life required during the 41-day deeksha (vow). 

• Appeasing Lord Shani: 

  According to one tradition, wearing black helps devotees suffering from Sani Doshams (effects of Saturn's malefic influence) and seeks Lord Shani's favor, as black is his associated color. 

• Practical reasons: 

  In some versions of the belief, black clothes are said to offer protection from wild animals in the dense forests around the Sabarimala shrine and provide warmth during the colder months when the deeksha is often observed.