Make a duplicate layer of the original image.
Have the duplicated layer in front, change the opacity down to about 50%.
Just enough so you can see both layers.
Erase everything in the duplicated layer that you don't want to be flat blue.
If you slip with the eraser, no biggie just use the paint brush and touch it up.
Once you have everything in that layeryou want changed, use the replace color tool.
Image > adjustments > replace color
If you don't like the results of that you could always use the Match color tool.
Typically I just play around with a bunch of different settings until I get a cool effect, or what I was looking for.
Good luck!
