Thanks @JamesStaddon for your critique of my tutorials, I fixed the links.
After you blurred the “dark spot” layer and changed it’s opacity, can you re-edit the layer? to make the dark spot smaller or larger?
Certainly! One way would be to simply scale the layer, make it large enough to cover the area you want to cover. Alternatively, you could take the brush tool again and paint more black on to your “dark spot” layer. You might have to blur it again to make sure that you’re not making too hard of an edge. I’ll update the tutorial to add those options.