In NDPR, Mark A. Wrathall reviews The History of Beyng (GA 69), translated by William McNeill and Jeffrey Powell.

But what could beyng be? It is something like an attunement, operating in the background of all worldly interactions, that governs how entities can show up. Heidegger's ontology is strongly relational -- that is, what something is is a function of how it relates to other entities. That certain relations stand out as definitive and essential, while others recede in importance, is a function of the "in-between," that is, the clearing traversed by the relations that unite entities to each other so that they can mutually define each other. Beyng is that "in-between," the essential, abyssal ground of the in-between" that sustains and structures relations.

So, then, blue is the color of beyng.  ¶ 7:24 PM