Neel or Ultramarine Blue works on the perception. What is does it curbs the undertones of yellow in the fabric while accentuating that of blue. It gives a neater and cleaner appearance to the fabric. It is an inorganic pigment. It would be interesting to find if someone is using the ultramarine blue for textile pigment printing. It has other uses apart from bluing of fabrics. It is used in the traditional Chikankari industry to trace the outline, on which embroidery is done. In this context it would also be interesting to find out if in Chikankari, neel is used both in Cotton and Tussar. And how to remove the neel on Tussar. You can view the full article in My Textile Notes