azular

To dye or color something blue — azular la tela, or historically, to add bluing agent to white laundry to counteract yellowing. A transitive, deliberate-coloring verb, as opposed to azulear (below), which describes something naturally taking on or showing a blue tint.

• El tinte azuló el algodón por completo. — The dye turned the cotton completely blue.
• Antes se azulaba la ropa blanca para que no amarilleara. — White clothes used to be treated with bluing so they wouldn't yellow.
• El artesano azuló la cerámica con óxido de cobalto. — The craftsman colored the ceramic blue with cobalt oxide.

Easily confused with azulear (to turn/appear blue on its own) — azular is what you do to something (transitive), azulear is what something does by itself (intransitive).

Often confused with azulear.