When something absorbs a specific wavelength or color in the spectrum, what you see is the opposite, or complementary color as long as that color isn't also absorbed. Blue and yellow are complementary colors, so when it filters out the yellow, you are left with the blue light. This is why sunglasses with a yellow/orange colored polarized lens give everything a bluish tint while glasses with a blue lens give everything a yellowish tint.

Thus, Cobalt blue glassware appears blue because it is absorbing/filtering yellow light.