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?
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.