The Sikh Empire - The Nihangs
The Nihangs or Akalis are a warrior sect that have always considered themselves as the Guru’s standing army, ready for battle at any time. They emphasize the martial tradition and their Nishan Sahib has always been distinct in appearance from other Nishan Sahibs.
The Nihang Nishan Sahibs seem to have had very little variations over time and have usually been a plain dark blue cloth without any emblems or pattern and sometimes with a thin border. Today at Nihang run gurdwaras in Punjab the dark blue Nishan Sahib can still be seen, with the only variation being the modern khanda emblem at its center.
The Dark Banner
The Nihangs have historically emphasized the color blue and early travellers noticed this distinction between the larger Sikh community and the Nihang sect when it came to their attitudes regarding color.
They do not tie on their heads red turbans from the dye of safflower. Most of them put on blue turbans. The wearing of kachhha (short breeches) is very common in this community. They are divided into two named groups. The first, comprising those who put on blue attire which Guru Gobind Singh used to wear at the time of battle, are designated Akali Sikhs (Sikh Akalia); the second, simply called Sikhs, do not follow any restriction on the colour of their dress.
Tashrihu ‘I Aqwam, James Skinner, 1825, 
This emphasis on blue has not only been reflected in the clothing worn by the Nihang sect, but also their Nishan Sahib which has historically been a dark blue. The Nihang interpretation of why their Nishan Sahib is blue according to the former leader of the Budha Dal, the largest faction within the Nihangs is as follows:
“Blue clothing has been ordained in the Rehat of Sri Guru Gobind Singh Ji. At the time the Akali Panth was created the colour blue was chosen. Our Nishan Sahib are blue, I do not know why this tradition of orange/yellow Nishan Sahibs has come about. We are the Panth of Akaal Purakh (Timeless Being) and blue is the colour we have been decorated with. For this reason where there is an encampment of Nihang Singhs a blue battle standard stands high, this is the symbol of our panth. So Nihang Singhs are the Akali Singhs, this is the panth of the Guru and this is the uniform of the panth.”
Jathedar Baba Santa Singh, Audio Recording May 1996 
As the Nihang sect has traditionally been a military order, no distinction has existed between religious / civilian banner and military banners. For the Nihangs they were one and the same. The dark Nihang battle standard displayed at Lichfield Cathedral in England and captured by the British during the Battle of Ferozeshah in 1845 differs from the other captured Lahore battle standards in this regard. While the Lahore battle standards were not used as Nishan Sahib’s, but as military banners, the Nihang battle standard was also their Nishan Sahib.
1. J.S. Grewal & Irfan Habib, Sikh History from Persian Sources, New Delhi, 2001