Monier-Williams: A Sanskrit-English Dictionary (p. 567)
nīlinī, f. the indigo plant Suśr. (-phala n. ib.); a species of Convolvulus with blue blossoms, I.; N. of the wife of Aja-miḍha Hariv. (cf. nīlī, nalinī).

Tamil Lexicon, University of Madras (p. 2314)
nīli: 1. Black hue; 2. Durgā; 3. Pārvatī; 4. A female devil; 5. Wicked woman; 6. Indigo plant; 7. Western Ghats blue nail dye; 8. Three-leaved chaste tree; 9. A poison fang of a snake; 10. Blue vitriol

Dymock, Warden, Hooper: Pharmacographia Indica (vol. I, pp. 406-410)
Indigofera tinctoria, Leguminosae
Fig. - Dyer's Indigo (Eng.)
Indigo, in Sanskrit Nila, a word which signifies dark blue or black, appears to have been known in the East as a dye and medicine from a very remote period. Its importance as an article of trade is indicated by the Sanskrit synonym Banigbandhu, or "trader's friend." It was probably exported from Cambay, Broach and Thana at a very early period, certainly from the latter port B.C. 30. What Dioscorides calls Indicon, and Pliny Vitruvius Indicum, was a blue pigment brought from India, and used both in painting and dyeing. When powdered it gave a black powder, and when suspended in water it produced an agreeable mixture of blue and purple. It belonged to the costly dye-stuffs, and was adulterated by the addition of earth. On this account, that which was soft without any roughness, and which resembled an inspissated juice, was esteemed the best. Both Pliny and Dioscorides speak of two kinds, one of which adheres to reeds, in the form of slime or scrum thrown up by the sea; the other was scraped from the sides of dye-pans in the form of a purple-coloured scum. The ancients considered Indicum to be astringent, and used it for ulcers and inflammation, and to cleanse and heal wounds. (See Beckmann's Hist. of Invent. II., p. 258, where the subject is fully discussed.) The early Arabian physicians identified Indicum with Nil, which they regarded as a kind of Indian woad. Ibn Sina calls it El-wasmah-el-Hindiya, and it was also called Idlim, which was an Arabian name for woad, as appears from a passage in Abu Hanifeh, who says: - "An Arab of the desert, of the Sarāh tribe, told me that the Idlimeh is a plant that rises upon a stem about a cubit in height, and has branches at the extremities of which are what resemble the blossoms of the coriander, and it (the plant) is dust-coloured." In Ibn Sina's time woad appears to have been superseded by indigo, as he describes wasmeh as wark-un-nil, or "leaves of the Nil." In the 13th century, Marko Polo relates that he saw Indigo, which the dyers used, made in the kingdom of Coulan or Coilum, and he describes the process for preparing it. Persian writers on Indian drugs state that before the time when the English began to cultivate indigo, the best kind made in India was known as Baiana, from the name of a place in the Shahjehanabad district where it was made, and the record of the cargoes of the ships which arrived in Holland from the East Indies in 1631, show that the first had 13,539 lbs. of Sirches indigo; the second 82,734 lbs. of Guzerat indigo; the third 66,996 lbs. of the same; the fourth 50,795 lbs. of Bajana indigo; the fifth 32,251 lbs. of Sirches indigo; the sixth 59,698 lbs. of Bajana indigo; and the seventh 27,532 lbs. of Sirches. The value of the indigo brought in these ships was at least 500,000 dollars.
The indigo plant was not known in Europe until the close of the 16th century.
Both Hindus and Mahometans consider the plant to have attenuant properties; they prescribe it in whooping-cough, affections of the lungs and kidneys, palpitation of the heart, enlargement of the spleen or liver and dropsy. Indigo applied to the navel of children is said to act upon the bowels; it is applied to the hypogastrium to promote the action of the bladder.
A poultice or plaster of the leaves is recommended in various skin affections, and is used as a stimulating application to old ulcers, haemorrhoids, &c. Indigo is applied to the bites and stings of venomous insects and reptiles to relieve pain, also to burns and scalds, and in Bengal is commonly applied to wounds, &c., of horses and cattle.
The plant has a great repute in some parts of India as a prophylactic against hydrophobia, so much so as to be known among the natives as "the dog-bite shrub." A wineglassful of the juice of the leaves is administered in the morning, with or without milk, for three days, to those who have been bitten by dogs supposed to be mad. People who have taken it inform us that beyond slight headache no disagreeable effect is produced, but that when a larger dose has been given it has proved purgative. In addition to the internal administration, the expressed leaves are each day applied to the bitten part as a poultice, Rheede, speaking of indigo, says - "viribus veneni obsistit." Ainslie notices the use of the root by the Hindus in hepatitis. It would appear that the wild indigo (I. paucifolia, Delile), is considered to have the same medicinal properties as I. tinctoria and its variety I. anil. For Roth's observations on the use of Indigo in epilepsy and other spasmodic affections, see Brit. and For. Med. Ref., July 1836, p. 244. His account of its physiological effects is as follows: - "Shortly after taking it, the patient experiences a sense of constriction at the fauces, and the impression of a metallic taste on the tongue. These are followed by nausea, and frequently by actual vomiting. The intensity of these symptoms varies in different cases. In some the vomitting is so violent as to preclude the further use of the remedy. The matter vomited presents no peculiarity except its blue colour. When the vomiting has subsided, diarrhoea usually occurs: the stools are more frequent, liquid, and of a blue or blackish colour. The vomiting and diarrhoea are frequently accompanied by cardialgia and colic. Occasionally these symptoms increase, and the use of the remedy is in consequence obliged to be omitted." Dyspepsia and giddiness sometimes succeed. The urine has a brown, dark, voilet colour; but Dr. Roth never found the respiratory matter tinged with it. After the use of indigo for a few weeks, twitchings of the muscles sometimes were observed, as after the use of strychnia. The seeds of these plants powdered and steeped in arrack or rum, yield a tincture which is used to destroy lice.
Cultivation and production. - Indigo is chiefly cultivated in Bengal in the delta of the Ganges, on those districts lying between the Hooghly and the main stream of the former river. The ground is ploughed in October and November after the cessation of the rains; the seeds are sown in March and the beginning of April. In July the plants are cut when in blossom, that being the time when there is the greatest abundance of dyeing matter. A fresh moist soil is the best, and about 12 lbs. of seeds are used for an acre of land. The plants are destroyed by the periodical inundations, and so last only for a single year. The cut plants are first steeped in water, when they ferment with evolution of CO2, the yellow liquor is then run off into another vat, when it is vigorously mixed with air by manual labour or machinery. By this means the leucindigo (white indico) contained in the solution is oxidised, and the indigo separates out as a blue scum which finally settles to the bottom. The supernatant liquor is then run off, and the indigo is boiled with water for several hours, pressed and dried. (Watts' Dict. of Chem.) Before it is perfectly dry it is cut into cubes three inches square; it is then packed up for sale. Indigo is one of the most precarious of Indian crops, being liable to be destroyed by insects as well as inundation of the rivers. It is generally divided into two classes, viz, Bengal and Oude indigo. Madras indigo is not much inferior to that grown in Bengal.