Human perception of intensity is proportional to the logarithm of the physical intensity. When light from an image enters our eyes, we do not perceive the changes in brightness proportionally, but rather proportionally to their logarithm. However, the real physical process of dispersion of light by a lens does not care about how our eyes and brain work. Which means that computer algorithms trying to simulate the effect of blurring a sharp contour should operate after translating first the intensity values to linear space. However, it seems they don't, and that is the origin of the fake dark stripe that appears after doing, e.g., a "photo blur" with Gimp on an image like this one:
This effect is explained by this youtuber here. And, while this usually goes unnoticed unless you are blurring a picture of the flag of Portugal, I wonder how to do a less unrealistic lens blur using Gimp. The stress is on the words "less unrealistic" because the subject seems to be quite complex, but at least a quadratic average of the RGB values seems to give better results in the examples shown on the YouTube video referenced above.
I am using GIMP 2.8.18 which is somewhat outdated.