Hello Pito,
First of all I am not an expert on this subject, but might be able to help you get it sorted out. On the other hand you might already know what I am writing here, in that case I am not much of help -;)
The approach you took is a possibility but it needs some understanding of the RGB colour model and how to create different colours in mixing RGB-colours will help a lot. If you search the internet you probably will find some information that will help you about mixing. Look for RGB colour wheel/model. Very often it will lead you to Photoshop explanations, but the principal is the same.
I was able to create a blue colour in one blown-out sky (but still a bit “artificial”), using the method of masking the sky. This all done on a jpeg-file,
However I don’t know if you are shooting I RAW or jpeg.
I shoot mostly in RAW, and I discovered that using Nitro, was able to retrieve blue sky with clouds (from blown-out sky) very efficiently and beyond expectations. I hardly managed that in other software.
If you are shooting RAW, I will gladly make some suggestions how I was able to do that (It’s a bit beyond the standard “retrieve highlights”). It was really a surprise to me by choosing the right workflow in Nitro.
Hth
Ben