Blue is a primary color, cannot be mixed. That's theory and might be semantics. This a photo of a palette with my blue that I make from Pthalo green and dioxyline purple. I put a bit of the purple and the green so you can see the pigments I use.There is also a bit of ultramarine blue on the top side for a comparison blue.
Joe Sosa described this phenomenon when I bought his Blue painting back in the early 1980's.