How do I make my curry sauce turn blue?

yup,food coloring is all I can think of as well.

depends on what else is in the sauce. If you have added turmeric you will have to deal with the yellow/orange color and adding blue might give you a green to brownish hue.

I can not imagine why you want a blue curry.

Add blue food colouring.

A suggestion is, add some food coloring to the yogurt or sour cream that your'e adding to the curry, If you add it straight into the curry it might turn a muddy / purple color.

That said, I'm unsure as to what color you will get adding it with the yogurt.

I can only suggest using blue food dye

Blue food coloring

leave it standing at room temperature for a few weeks