Sambucus nigra or European Elderberry as its commonly known is a flowering plant in the family Adoxaceae native to most of Europe and North America.
Black Elderberries have been used for centuries as a natural treatment against common cold and flu like symptoms.
Elderberries are a good source of a wide range of nutrients including vitamins(A, B-Complex, C & E), minerals such as potassium, calcium, magnesium, copper, zinc and iron.
Elderberry contain high quantities of the flavonoid anthocyanins, which is responsible for its strong antioxidant capacity and also considered to be the active constituents of the fruits.
Other phytochemicals found in Elderberry include carotenoids, phytosterols and polyphenols.
Modern clinical research has found that Elderberry has diverse pharmalogical activities including antioxidant, anti-viral, anti-inflammatory, immune modulating and anti-diabetic effects.
Black Elderberry, European Elderberry
The main active constituents of Elderberry are flavonoids such as quercetin and rutin, and anthocyanins identified as cyanidin-3-ambubioside and cyanidin-3-glucoside.
Common Cold, Influenza
Evidence-Based Health Benefits
Reducing Cold & Influenza Symptoms
Collectively the evidence obtained from across five clinical studies involving 936 adults indicate that mono-herbal preparations of Sambucus nigra L. berry (S.nigra), when taken within 48 hours of onset of acute respiratory viral infection, may reduce the duration and severity of common cold and influenza symptoms in adults. 
Interestingly, a non-travel related clinical trial just revealed that a combination of echinacea herb and root extract supplemented with elderberry (Sambucus nigra L.) can be as effective as the conventional antiviral medicine oseltamivir for the early treatment of influenza.
Two clinical trials using a liquid elderberry extract (Sambucol®, Israel) showed a reduction in symptoms and duration of influenza infection.
A pilot trial with elderberry extract lozenges (HerbalScience, Singapore) also confirmed a beneficial effect on severity and duration of cold and flu like symptoms. 
Effective Treatment For Upper Respiratory Symptoms
A meta-analysis of randomized controlled clinical trials published 2019 quantified the effects of elderberry supplementation on upper respiratory symptoms and evaluated moderators including vaccination status and the underlying pathology.
The meta-analysis concluded:
Supplementation with elderberry was found to substantially reduce upper respiratory symptoms.
The quantitative synthesis of the effects yielded a large mean effect size.
These findings present an alternative to antibiotic misuse for upper respiratory symptoms due to viral infections, and a potentially safer alternative to prescription drugs for routine cases of the common cold and influenza.