What color do blue and white make?
Blue and White make powder blue.

Blue and White make powder blue, #cad7e6. That is the result in equal parts, mixing pigment rather than light - paint, ink and dye all behave this way.
Equal parts blue and white · #cad7e6
Change the ratio
Very little of a mix is ever half and half. Shifting the balance moves the result along the range between the two colours.
3 parts blue, 1 part white · #7a9bbf
Equal parts · #cad7e6
1 part blue, 3 parts white · #dfe7f0
Lighter and darker
With white added - a tint · #dfe7f0
With black added - a shade · #59697a
Why it works
Adding white to blue makes a tint. The hue does not change - only its lightness does, which is why you get powder blue rather than a different colour altogether. Keep adding white and it keeps moving towards plain white.
Frequently asked questions
What color do blue and white make?
Blue and White make powder blue, #cad7e6. Mixed in equal parts that is the result you get with paint, ink or any other pigment.
How do I make the result lighter or darker?
Add white for a tint - here that gives beige. Add black for a shade, which gives grey. Add white a little at a time, because it takes far less than people expect to shift a mix.
What happens if I use more blue than white?
Three parts blue to one part white gives sage. Reversing it to one part blue and three parts white gives beige instead.
Does this work the same way on a screen?
No. Screens mix light additively, so red and green light make yellow there. These results are subtractive - paint, ink, dye and pigment - which is what people mean when they ask what two colours make.