www.sweetsunnah.com has this about the medicinal 'black seed' -- This small but incredibly healing herb is known throughout the world by many names such as black cumin, kalonji, blackseed, swartzcummel, Nigella sativa, fennel flower, habbat al barakah, sinouj, black caraway and habba sawdah.
You see how confusing this kind of issue is! Panfusine, thanks for the on-the-ground view; I knew we could count on you!
Can we hear from the questioner? What are you looking for/why?

The nigella sativa you mentioned is the dark coal black one (kalonji) used in Indian cooking, although you bring up another spice, black cumin which is referred to as kala jeera (or shahi jeera), (used, but not so commonly..) Wiki has the perfect page comparing the two..http://en.wikipedia.org/wiki/Shahi_jeera

Yet another: black cumin seed, nigella sativa, is commonly used in Egypt as a medicinal herb, available here as an oil, liquid or encapsulated. This article lists ancient and traditional uses: http://www.muslimhealthnetwork.org/ls_black_seed.shtml
I don't know if it has culinary uses, but it's likely that it does. There is recent research on it. (Is this the species of nigella used in Indian cooking?)

Black seeds: 4 candidates, Nigella, sesame, poppy seeds, & black mustard. check your local Indian grocery stores. the Indian names are Kalonji (nigella), til (sesame) Khus-khus (poppy) & Rai (mustard).

There is one other candidate, referred to as tukmaria (would be labelled as basil seeds) that looks similar to chia seeds & develops that gel like coating when soaked. Its used for a dessert called Falooda

what is the recipe? I have Black sesame seeds as well as poppy seeds. I do not currently have Nigella here at WHole Foods Market in Rockville. but any good specialty spice shop should have Nigella Seeds

Anyhow, you'll find them in Indian stores or Middle Eastern ones. Perhaps Wholefoods might stock them but not living in the US, I can't say for sure

Are you thinking of Nigella seeds (which are commonly referred to - wrongly as black onion seeds) and are common in Indian recipes?....or are you thinking of black sesame seeds?