I've never seen a black basil. But I do know that purple basil (and things like variegated rosemary or golden oregano) will often revert to its natural green state eventually, though some varieties are less prone to this than others. It's in the plant's genes to be green, and the only thing you can really do about it is to keep getting rid of the new green growth.

one word. L - i - g - h - t. :-)

More to do with the unstable purple pigments in basil. There are some varieties of purple basil which are less likely to revert to green.

Jamus,
one question....lack of, or too much? I'm presuming the latter, because of more chlorophyl production.....???

wondering, mudlark

No sorry mudlark, I didn't explain myself properly. Plants which produce pigments in leaves generally do so to protect themselves from excessive UV in sunlight. Many pigmented plants come from tropical or sunny climates. When you grow these plants in low light conditions the production of red or purple pigments (anthocyanins) reduces to maximise light reaching the photosynthetic organelles. For instance I grow Laksa leaf or Vietnamese coriander, which has a distinctive purple marking, arrow on the leaves. I have a plant in a sunny position and the leaves are dark green with strong purple markings. I also have it growing in my pond / bog garden which is under the oak tree. In this position the leaves are paler green and completely devoid of the purple arrow. Superficially you'd think it was a different species. However the flavour is still pretty good, and I think it's more tender!

Thanks Jamus for the explanation but my basil is planted in full sun so it must be some other misterious problem.

Basha - that's more interesting. I've seen that happen too, especially with variegation. Variegated plants reverting to non-variegated on the same plant. The original 'spot' is caused by a genetic mutation which is selected by the nursery person and propogated to produce a variety. As the plant grows the cells divide at the growing tips (mitosis). In each cycle of cell division the DNA is copied and if the mutation is corrected or reversed by chance in one of these billions of cell divisions that shoot (and every shoot that comes after that shoot) will carry the 'new' variety, ie. green instead of purple. If you cut off that shoot back before the switch you'll get rid of the sport, but it's likely to happen again and again. Maybe another plant would be less likely to revert, because all plants have genetic variation.

Having said allllll that!
It could be something as simple as pH or nutrition in the soil?

Thanks Jamus....very interesting.

cheers, mudlark

thanks Jamus , very good explanation.I am going to buy another black basil and see what happens....