Personally, I feel like modding in cobalt, it's a nice color.
Yeah, I've been considering adding in cobalt glass, myself. The refining process for cobalt is pretty toxic, producing an equal amount of arsenic, but we ignore that for other metals, so it's not much of a stretch to ignore it for cobalt. I don't think I'd want to mod in any other uses of cobalt for producing items...it's sometimes useful in minute amounts in alloys, but, reading between the lines of the Wikipedia article, I suspect it's too hard and brittle for practical use as a pure metal.
As a pigment, cobalt blue glass is called "smalt", chemical formula CoKSi, it's also sometimes crushed into powder to make a blue pigment for painting (if anyone wants to mod in painting). The real life reaction is cobalt oxide + pearlash + sand = blue glass (and presumably some stuff that gasses off).
The (non-glass) pigment "cobalt blue" involves aluminum, and is a bit unrealistic for dwarves to be producing.
So it wouldn't be that hard to imagine dwarves making it: smelt some cobaltite to cobalt, if you want the reaction could also realistically produce 1/10 iron or 1/10 nickel (perhaps as a random chance), then cobalt + pearlash + sand at a glass furnace to produce blue glass.
I'd also want to add in 'ruby gold' glass, but I'm not sure it's realistic...it apparently requires aqua regia, a mixture of powerful acids not produced until the renaissance, in order to prepare the gold for integration into the glass.