In Chinese Glazes, we learn from Nigel Woods that the cobalt used for underglaze blue & white underglazes and blue glazes came in a range of chemical compositions and grades of purity. Thus, there are many shades of blue due to the quality of cobalt-containing stone as well the overlying glaze.
In the same book, Nigel presents a lovely Chinese blue stoneware glaze which, in addition to cobalt, contains iron and manganese “impurities”.
In fact I’m personally not fond at all of glazes and underglazes containing only cobalt as a coloring oxide. Pure cobalt often comes out as a garishly blue color. In the triaxial blend below, I take a nice clear glaze (Sue’s Clear) with added 1% Cobalt Carbonate. Then I blend with 1.5% Red Iron Oxide (bottom left) and 1.5% Manganese Dioxide (bottom right). The resulting colors on the bottom row are much more pleasing to my eye.
The full image can be viewed here: http://www.derekau.net/wp-content/uploads/2014/12/BLUE_TRIAX_ALL.jpg