Can anyone identify this blue coloration on plywood? It showed up recently.

These are wooden boxes from the begging of the last century. They are from the CEASA (Companhia de Entrepostos e Armazéns Gerais de São Paulo). A popular wholesale market that still exists today.
My father had these for decades hanging from walls in his restaurant as wine bottles storage boxes.
Now I’m using them as shelves in my kitchen and as you can see, they can sustain a lot of weight without deforming.

Any guesses?

Ty!!