If you’re tasting it in Mexican and Indian food, it’s probably cumin. But cumin seeds are shaped like moon-like pieces of rice. Or if you know what caraway looks like, from rye bread, cumin looks like that, except slightly different color. Cumin is also one of the main flavors in Tex-Mex style chili.
Coriander is one of the other predominant spices in Indian food – I’d say cumin & coriander contribute the bulk of the curry flavor. Usually, though, both cumin and coriander is ground before being put into the food.
Little black/brown seeds in Indian food can be mustard seeds, black onion seeds (actually, nigella), or even poppy seeds. Oh, and possibly cardamom seeds.
Asafoetida (aka hing) is a very potent spice that smells something like rotting onions. It’s very tasty gently used, but can easily overpower a dish if you’re not careful. It comes sold as a powder or a gum. It’s used in some dishes in that part of the world, but I wouldn’t call it omnipresent, like cumin and coriander.
What is odd is that you say the taste is present in most Indian food you’ve tasted, but not in curry powder. My best guess would be cumin, if not for this fact. Since curry powder describes a wide, wide mix of spices, it’s just possible that the Indian food you’ve been eating just has a lot more cumin in it than whatever curry powder you’re using.
The second guess I would have is the cardamom–it has a very distinct taste, in the ballpark of “sweet” spices like cinnamon, cloves, and nutmeg, but a different flavor. That could be more present in Indian restaurant dishes than in curry powders, but I don’t usually encounter it in samosas.