I think those signs are partly intended as a marketing tool. On the one hand they tell the honourable Gold members: keep staying and one day this space could be reserved for you. On the other hand if offers some surplus value to the Diamond membership, without any further costs involved.
Yet I haven't heard anyone mention they saw the new signs at a full service Hilton. According to the
hotelsigns website, the white-on-black ones are meant for Curio, Hilton, DoubleTree and Tapestry.