Ok, looking for some painterly advice here...
This lovely MS page at V&A (dated 13th c, Germany page from a Gradual) has what I'd call a peacock? cerulean? blue colour in it. It may or may not come across in your screen, but trust me that when I viewed it at the museum - it's not the conventional lapis- or azurite-sourced blue.
link to a vendor page for palette - your screen's pixelage may vary).
To date, I've tried to stick to colours that I know have precedent in period, even if they are now synthetic versions ie. W&N ultramarine for lapis; alizarin for cadmium; vermilion for...vermilion.
It's not perfect, but to date it's served me very well, and I've rarely found a colour I couldn't reproduce, til now.
So what pigment do you think created this colour?
What would be the best approximation in gouache?