Turquoise turns dark green and almost black just from normal contact with body oils. If it turned black after coming back maybe you wore it more often and that made it darken quickly.
It is a very porous stone. Wearing it doing dishes would also expose it to oils. Some stones are treated to prevent color loss and stabilize it. Some higher quality stones will resist it. Some of the coloration can be restored by buffing the stone, that is usually done by a professional who knows what to use.
That is why the stone darkens.