There is no evidence of kalonji seeds being useful in the treatment of COVID-19 and the World Health Organisation has not recommended it.
National Center for Biotechnology Information(PubMed) has published Central (PMC) research which says that kalonji seed and oil extracts are used for the treatment of malaria induced in mice. It further mentions that kalonji seeds (Nigella Sativa) have effects against skin diseases, jaundice, gastrointestinal problems, anorexia, conjunctivitis, dyspepsia, rheumatism, diabetes, hypertension, intrinsic haemorrhage, paralysis, amenorrhea, anorexia, asthma, cough, bronchitis, headache, fever, influenza and eczema.
World Health Organization (WHO) has not recommended kalonji seeds for the treatment of COVID-19. There is no evidence that current medicine can prevent or cure the disease. WHO does not recommend self-medication with any medicines, including antibiotics as a prevention or cure for COVID-19.
The COVID-19 pandemic has given rise to a lot of potentially dangerous misinformation. For reliable advice on COVID-19 including symptoms, prevention and available treatment, please refer to the World Health Organisation or your national healthcare authority.