The distinct aroma of Bengali cuisine comes from the blend of spices known as panch phoron. Panch means “five” and phoron is “flavor”. Panch phoron is a blend of flavorful seeds: the green fennel, yellow mustard, black nigella, golden fenugreek and brown cumin.
The ingredients are generally added in equal proportions, though this can vary according to taste.
Nigella seeds are about the size of sesame seeds. The seeds appear often in East Indian Cooking. Their flavor is enhanced by heating or baking.
If you can't find nigella seeds, I have given a substitue that will give you a good likeness of the oniony, sour taste. If you are looking for the color, you can add some black sesame seeds.