Judging just how far you blend the white that you're adding to create the clouds into the blue of the sky comes with experience. But one of the advantages of painting wet-on-wet is that if you add too much white and the sky blue becomes too light, you can either scrape it off or add more blue.
Blend the white in too little and you end up with cotton-wool style clouds that sit on top of the blue sky, not in it. Blend the white in too much and you end up with pale blue sky without any discernible clouds. It's a bit like Goldilocks trying the bowls of breakfast porridge... through trial and error (experience) you get the result you're after.
