Why is my blue a dull purple?

[Robert]

The sky dissapears if you don't get the contrast just right.

All you need is photoshop, and you can remove the sky completely and throw in a blue to white gradient. This will ensure it is spot on
Either that, or you could put on a layer mask, then gradient, and this will ensure that it fades out to white.
No need to mess with the contrast this way as it would affect the rest of the image.