They got bigger and spreaded to the whole plant. Any idea what could be the problem and how to fix it? I couldn't find any insect on the plants though. And I placed them in a shadow spot in my balcony.
It looks like Fungal Leaf Spot. The tiny black spots are called 'fruiting centers', that is where the active part of the fungus makes and then sends out new fungal spores.
The easiest way to control common fungal leaf spots is to spray the leaves with Lysol. Lysol is generally safe to use on most plants, however it is always advisable to spray a 'test leaf' first. If the test leaf shows no adverse reaction after 48 hours, then spray the rest of the plants.
The Lysol, or any other fungicide, won't actually 'cure' the fungal leaf spot. What it does is shut down the fruiting centers from making new fungal spores. This will prevent the fungus from spreading to new leaves. So you will have to spray the plants a couple of times a week until all the affected leaves are gone.
Remove the most infected leaves as soon as possible. Once the plants start producing new, unaffected leaves, you can remove the other infected leaves a few at a time.
If you harvest any unaffected leaves while you are still spraying the plant with Lysol (or other fungicide), don't forget to wash the leaves thoroughly before consuming them.