Nigella seeds
Small, matte black, angular seeds with a slightly bitter, oniony and peppery flavour, familiar from Turkish flatbread.
Nigella seeds are sold in Turkish, Persian and Indian grocers, often also in the spice aisle of larger supermarkets or by mail order; in Indian shops they are labelled kalonji and sometimes called onion seeds. Despite names like black cumin, they are not related to caraway, cumin or Persian black cumin (zireh siah), which is a different plant.
Substitute:
There is no true substitute. Black sesame copies only the look, not the bitter, oniony note, and caraway or cumin taste clearly different. If in doubt, leave it out.