It's just on the basil, my other herbs growing in separate pots beside it seems fine. And just the bottom clusters of leaves are affected, the top ones look ok (so far). Some "expert" at the gardening store told me that the black spots could be mold and the orange bugs are aphids, which makes sense since a lot of my leaves have holes in it. He suggested I get some 3 in 1 spray which gets rid of the mold as well as kill the bugs, but since the basil is for eating, I am wary of using chemicals.

Do you think my new potting soil could be the cause of it? Or is it just something natural to herbs (even if I am growing it indoor)?

The mould sometimes occurs because of aphid poop, get rid of the aphids, get rid of the mould maybe? I'd remove all those leaves to be sure. It's unusual in my limited experience to have aphids show up indoors out of nowhere.

You can use a spray from water and dish soap to get rid of aphids. Safe and saves you money. You can google to get the proportions. I usually try to also wash the aphids off if I can with just water. Works like a charm.

Pretty easy. You can get seeds from any supermarket. I got mine in Food Basics, a box with 3 seeds (basil, parsley and chives). Those turned out ok. You just need to put them by the windowsill and water them everyday. Pretty easy to grow. The Thai Basil with the problem is bought already grown from Loblaws. Just water and sunlight (and pinching them every now and then) and they would bloom. Very easy to do actually (apart from this bug problem).

those seeds comes with pots. you can grow on those for at least 6 months to a year before you have to move them to a larger one. My window faces west so they get plenty sunlight, but I dont think they need *a lot* of sunlight, as they survived well through the winter. Other experts here may have more idea on the lighting requirement.