When looking at the human evidence in particular, nigella sativa appears to have a general health protective effect of a very small magnitude. It is able to reduce triglycerides and glucose when elevated and can normalize cholesterol metabolism (although not HDL-C) as well, but the amount of benefit it confers is too small for it to be considered a good intervention. It's also modestly effective as a general anti-inflammatory agent and may be useful in the treatment of rheumatoid arthritis.
Nigella sativa shows more promise as an immune booster (no evidence on rates of sickness, but it seems to increase the ability of macrophages to attack invaders) and an anti-allergic compound (with efficacy against allergic rhinitus, asthma, and eczema). One study that used nigella sativa alongside a proton pump inhibitor noted that the seed extract was potently anti-bacterial against helicobacter pylori with a comparable potency to the reference drugs used.
Lastly, in men with central obesity, the seeds were noted to greatly improve symptoms of obesity, such as loss of libido, forgetfulness, high appetite, aches and arthalgia, sleeplessness, and laziness. But further evidence is needed to confirm these effects.