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First, I have no trouble blurring the images. However, that generally isn't the proper method to deal with the noise in the images. Unfortunately without the EXIF info and an understanding of your workflow, I can't conjecture why they have such a high noise level. However, when I've encountered this before it's often been with under-exposed images that were pushed to recover a lighter density - trying to mine image data from shadows.
What you need to apply is Filter>Noise>Reduce Noise and then fiddle with the sliders. However, that can also somewhat trash the image detail. My preferred method to remove the noise but retain the detail is to make a duplicate layer and apply the Reduce Noise filter to the duplicate layer. Then I erase the duplicate layer over the detail area.
Here's a section of the red sky scene made using the above Reduce Noise duplicate layer with the ground portion of the duplicate layer erased. The next image if the skyline with a Reduce Noise duplicate layer with the buildings erased, applied twice.


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