Mulberries of the Morus nigra species are generally regarded as the most flavorful. Laboratory tests consistently show them to be higher in sugars, antioxidants, and organic acids (component of flavor) than other mulberries.

Here are some partial results comparing them to Morus rubra (also regarded as being very flavorful). 

From: "Phytochemical and antioxidant properties of anthocyanin-rich Morus nigra and Morus rubra fruits", Mustafa Ozgen, Sedat Serce, Cemal Kaya, 2008

Morus nigra fruit
Old Black Mulberry (Morus nigra) in Lebanon

This stone wall was erected around an old Morus nigra mulberry in Lebanon.

Living for hundreds of years in favorable locations, Morus nigra mulberry trees often hold a position of reverence in European settings. Mulberries in general command little respect in the United States, but Morus nigras tend to be sought out over other mulberry rivals. As they are more desired, unwittingly or not, sellers quite often have other cultivars of mulberry labeled as Morus nigra. Look for named cultivars when purchasing: 'King James I' and 'Noir of Spain' are your best bet; 'Persian' and 'Black Beauty' may or may not be Morus nigra, depending on the nursery. The 'Dwarf Everbearing' is NOT Morus nigra.

Morus nigra CAN be grown from seed; the trick is to find the correct seed, stratify them for three months, plant and wait for up to ten years for fruit (also, about a third of those plants will be male without fruit). This author made eleven separate online purchases in an attempt to obtain Morus nigra seeds; not a single purchase resulted in obtaining Morus nigra seed.

Morus nigra seedlings

Morus nigra seed size

Morus nigra seeds are 3 to 4 mm in length; compared to Morus alba, Morus rubra and Morus macroura, which range from 1.5 to 2.5 mm in length.

Leaf-spot disease on Morus nigra

Diseased Morus nigra (before spray program)

Morus nigra appears more susceptible to fungal and bacterial spot diseases to a greater extent than other mulberry species. These organisms have their highest rates of infection in a warm and wet environment. Not many are so foolish as I to attempt to grow Morus nigra out of its ideal range (I'm in Northern Florida), but for those who wish to give it a try, here is a brief summary of what I believe is necessary to maintain healthy growth.

Here in the United States, Morus nigra does well in the hot, but dry Southwest, and also does well in the wet, but cool Northwest coastal region, or wherever yearly temperatures have a typical maximum range of zero to 85 degrees F. In all other regions in the United Stated (in USDA Plant Hardiness Zones 7 to 10) Morus nigra struggles to survive. Here they most likely will require fungal, and perhaps, bacterial sprays and a good fertilization program to flourish and be productive. In the spring, examine leaves frequently and start spraying at the first sign of leaf spot disease. Under warm (above 85 degrees F) and humid conditions, weekly spraying may be necessary, and even more frequent during periods of excessive rain. A safe, non-toxic spray  should be considered, especially through harvest. After harvest, switching to a more robust (not so environmentally friendly) spray may be necessary to contain disease progression. Be sure to spray the leaf underside. Keeping the plant well fertilized and watered will help keep it healthy and growing.

Update: Although it is important, if not critical, to use fungicide sprays initially, it appears that Morus nigra has the ability to adapt to a climate and its pathogens, as less disease occurs as the plant ages.

This video exemplifies typical disease problems. A plant pathologist felt it had a Fusarium fungal and an unidentified bacterial disease.

Disease-free Morus nigra May 2021.jpg

Morus nigra mulberry now disease free. No fungicidal sprays have been necessary on my North Florida Black Mulberry for over two years.

Fruiting Black Mulberry (Morus nigra)

Morus nigra: Note the large dark buds       and berries that hug the branch.

Morus nigra staining hand

It's difficult to pick ripe nigra berries without staining your fingers.

Black mulberry (Morus nigra) grove in Lebanon
True Black Mulberry (Morus nigra) at cottage in Lebanon

Morus nigra mulberry trees commanding respect in Lebanon.

Crushed Morus nigra fruit showing seed size

Crushed Morus nigra fruit:

Seeds are seldom found in Morus nigra fruit in the United States, most likely due to the lack of pollinators, since plants bought here are usually dioecious females. This single mulberry fruit with many seeds was from a tree growing in Bulgaria.

Morus nigra videos on YouTube

Here is a list of YouTube videos which appear to be actual Morus nigra videos. Many videos on YouTube, perhaps most, showing Morus nigra or one of its synonym names, are something other than Morus nigra.