My very limited understanding says that different colors of copper-based sparkly stuff went in (lutz or goldstone or aventurine or whatever it could be called) and that led to different colors coming out of the furnace -- including different shades of what we call oxblood.
In the olden days they had a lot of different names for the different shades. Representing different kinds of animal bloods (which they knew about up close and personally) or sometimes less gruesome names, such as peach bloom, crushed strawberry, crimson and liver.
Sooooo ... that covers some of the different opaque shades which came out. In your photo, your blue-gray is not looking opaque to me but maybe it could still be connected with the lutz they used
.... and with that my conjecture and stalling ends and I wait for correction, clarification, additional info from someone who knows!