The Spanish for “blue,” azul, is originally an Arabic word referring to a particular type of valuable blue stone, the lapis lazuli. In Spanish, the word degraded over time, and the l- was lost (as though it was the the french l’ for “the”) and we were just left with azul for just “blue.”
The English for azure — which is really just a shade of blue! — comes from the same root, although azure still retains a luxury connotation that was lost with the simple blue implication of azul in Spanish.
Many languages, including Spanish, have an ‑l- and ‑r- shift, where, over time, the ‑l- and ‑r- sounds are swapped. We see this here, as the a‑z-l root of azul maps to the a‑z-r root of azure.
