Yes, in general Kalonji seeds (Nigella sativa) can be used by Jains.

From a Jain-diet point of view:

• Kalonji is a seed of a plant, not a root, tuber, mushroom, or fermented product.
• It is used in very small quantity as a spice (like jeera, rai, saunf), so the himsā involved is relatively less compared to eating underground vegetables.
• It is not in the category of onion–garlic, nor in the clearly prohibited items mentioned in Jain śāstras.

So, for most Śvetāmbara and Digambara lay Jains following normal Jain vegetarian rules, using kalonji in cooking (tadka, pickles, etc.) is generally acceptable.

Only two cautions:

1. If someone follows a very strict vrata and has been advised by their guru to avoid strong/spicy items, they may choose to avoid it personally.
2. If used in large medicinal doses, it should be taken carefully under proper guidance, avoiding dependence or misuse.

But for normal home cooking in small amounts, Jains can use kalonji seeds.