A missing-looking blue histogram usually does not mean the blue channel is gone. It often means the blue data is heavily bunched toward the right side of the histogram, so the rest appears compressed. In practice, that indicates a strong blue cast, especially in the highlights.

A UV filter is not the cause here. On modern digital cameras, a UV filter generally does not change exposure or color in a meaningful way; it is mostly used as lens protection.

To correct the images in GIMP, adjust color rather than looking for lost data:

• Use Levels and move the midtone slider, and possibly the black/white point sliders, to restore contrast.
• Use Curves on the blue channel to reduce blue in the affected tonal range.
• Use Color Balance to warm the image and neutralize the blue cast.

The exact correction depends on the specific photo and your preferred look, but the key is that the blue channel is likely overrepresented, not absent.