Here might be a place to start. Magnesium mattes with some calcium and cobalt can go pretty vivid indigo without having to get toxic.
https://glazy.org/recipes/16778
That is most likely a stain or a very saturated cobalt. Most saturated cobalts are dark blue, I have never seen such a light blue like that with cobalt.
Look at these mason stains below.
https://www.theceramicshop.com/product/8693/delft-mason-stain-6320-1-lb/
https://www.theceramicshop.com/product/8703/copen-6368-mason-stain-1-lb/
https://www.theceramicshop.com/product/8682/vivid-blue-6306-1-4lb/
Note: Stain are usually between 3-10% in glazes. If you want to make (say a 5000g batch) you are better off getting 1 lb of stain, many people (myself included) often get 1/4 lb and it's a few grams off, and there is not enough for a batch.
Hi Imogen and welcome to the forum.
Cone 7/8 is an unusual cone to fire to, is this the range in which your clay matures or are you using one that is listed as having a broad firing range or is it a cone 10 clay? For functional work you want the clay to be vitrified.
Looks like a cobalt blue satin or semi matte glaze. Using a clear or white gloss liner glaze is a good idea if you want to avoid the possibility of the cobalt leaching and cutlery marking with a matte glaze. You can do a couple home tests to rule out leaching glazes, there is also the option of having glazes lab tested. Glazy has quite a number of recipes, fill in your search terms on the left side of the screen. Cone 7/8 might not have many though.
Having plenty of both silica and alumina in the glaze, firing it to maturity, not overloading most colouring oxides or fluxes and testing the glazes before putting them to use on functional pots are all important. Mastering Cone 6 Glazes is a great book for a fairly basic intro to making glazes and well worth reading if you are wishing to learn more about what makes a durable glaze.
