The blue second layer here is 5x20 cone 6 clear with 2% cobalt oxide and 5% rutile. The base black glaze is 5x20 with 4% Mason 6666 stain.
G1214M - Original Cone 6 Base Glossy Glaze
A recipe developed by Tony Hansen in the 1980s. Its was popular because of the simplicity of the recipe and how well it worked with chrome-tin stains.