Left is Plainsman Zero3 stoneware fired at cone 03. Middle is Polar Ice fired at cone 6d. Right is Plainsman P600 fired at cone 10R. The same black and blue underglazes are used on all three, but each has its own transparent glaze (left G2931K, middle G3806C, right G1947U).
A ceramic compound meant to be applied to green or bisque ware and covered using a transparent overglaze. There are good reasons to make your own underglazes if you are in production.
Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it.