= sense B. 1c.

spec. A blue powder used to combat yellowing, and hence preserve the whiteness of garments and fabrics in laundering, by adding a slight tint of…

Powdered smalt.

A substance that imparts a blue tint; esp. laundry blue (see blue, n. 1c).

= powder blue, n. 1.

A bag containing laundry blue (see blue, n. 1c), used to whiten fabrics and formerly also to treat insect bites and stings.

(a) n. a type of laundry whitener, now chiefly used as a brightening agent (cf. blue, n. 1c); (also) the clear cobalt blue colour of this…

The detergent matter used in scouring wool.

Reckitt's blue sold and used in a bag; an example of this.

= Reckitt's bag blue n. at sense 1b.

causing the loosening of dirt from cloth during washing; also as n., such a substance.