Structureless Black

Structureless Black  - In most cases it is due to melanin high in the epidermis or stratum corneum. Now that melanin can be in keratinocytes in very dark seborrhoeic keratoses or in atypical pagetoid melanocytes ascending through the epidermis in melanoma but also in Reed nevi where highly pigmented nests of cells are pressing from the dermis thinning the overlying epidermis. 

Old haemorrhage  looks black. The commonest example of this is the talon noire with subcorneal frictional hemorrhage. 

Thrombosed angiokeratoma  It also is black structureless . It may be seen adjacent to other non thrombosed purple vascular lacunae or as a larger thrombosed lesion where it may be confused with a nodular melanoma.

Nevus   Reed nevus is the most likely to show structureless black but also numerous peripheral pseudopods.

Melanoma   A Nodular melanoma can be black but it is equally often pink. Here it is the thick mass of pigmented melanocytes in the dermis that gives the black colour but there is often a blue tinge as well from the deeper dermal melanocytes.

Image courtesy of Alan Cameron in Dermoscopy Atlas 

Seb K   The really black seb k is the melanoacanthoma where there are melanocytes with excess dendrites putting  lots of pigment into the overlying keratinocytes in the epidermis.