He could be cold, stressed, sick or just turning dark to absorb more heat from the light. If he is in a cage that is too dark, chameleons will often be dark. It is normal for them to go through different colors and shades throughout the day. As you keep him longer, you will learn what is normal for him. Spend the time to just observe him and find out what makes him tick. If he stays that dark color all the time, then it is worrying. If it is only for short periods of time, it is probably completely normal. In general, dark spots suggest stress but not always. I know, clear as mud!
If you think he is darker than you think he should be, look at your husbandry first. Is the temperature correct? Does he have enough light, not just UVB light but regular UVA light. Most chameleon cages are kept too dark. Does he have enough cover to feel safe?
Your cage looks pretty barren. You have very few horizontal branches up top where he wants to be. If you start thinking like a chameleon, you will realize that your cover isn't very usable for him as it is in big masses of tangled fake plants. You can give more cover by taking the strands of fake plants and spreading them out along the branches. Zip ties are your best friend. You can also buy a big sword fern, divide it into a lot of smaller pots and zip tie those pots to the branches to make a fake tree using living ferns as the leaves rather than the plastic plants. With all-screen cages, I put a big potted plant that goes all the way to the top of the cage in the bottom in the middle and use that plant to hold oak branches from my trees to anchor the branch framework. Ficus and schefflera are both cheap and good plants to use. A really big pothos works, but it is difficult to hand a pothos in a screen cage.
