I just took a photo with manual focus and blurred as much as I could a light, but instead of getting a uniform colour, there are dots, like dust :
Do you know what's causing the darker dots?
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Since you're looking at the pixel level, these artifacts are too small to be dust. It's also unlikely that dust would be outlined sharply when you're using a wide aperture.
I think it's most likely that they are random shot noise combined with demosaicing artifacts, possibly then also smoothed out by denoising.