What is "thumb blue"

Thumb \Thumb\, n. [OE. thombe, thoumbe, [thorn]ume, AS. [thorn][=u]ma; akin to OFries. th[=u]ma, D. duim, G. daumen, OHG. d[=u]mo, Icel. [thorn]umall, Dan. tommelfinger, Sw. tumme, and perhaps to L. tumere to swell. [root]56. Cf. Thimble, Tumid.] The short, thick first digit of the human hand, differing from the other fingers in having but two phalanges; the pollex. See Pollex. Upon his thumb he had of gold a ring. --Chaucer. Thumb band, a twist of anything as thick as the thumb. --Mortimer. Thumb blue, indigo in the form of small balls or lumps, used by washerwomen to blue linen, and the like. Thumb latch, a door latch having a lever formed to be pressed by the thumb. Thumb mark.

1. The mark left by the impression of a thumb, as on the leaves of a book.
   --Longfellow.

2. The dark spot over each foot in finely bred black and tan terriers. Thumb nut, a nut for a screw, having wings to grasp between the thumb and fingers in turning it; also, a nut with a knurled rim for the same perpose. Thumb ring, a ring worn on the thumb. --Shak. Thumb stall.

   1. A kind of thimble or ferrule of iron, or leather, for protecting the thumb in making sails, and in other work.

   2. (Mil.) A buckskin cushion worn on the thumb, and used to close the vent of a cannon while it is sponged, or loaded.

      Under one's thumb, completely under one's power or influence; in a condition of subservience. [Colloq.]

n. indigo in the form of small balls or lumps, used for blueing linen etc.