Is this black spot on my back serious? (photo)

What the picture depicts is a possible congenital compound melanocytic nevus that shloud be evaluated by a dermatologist.

The fact that your mole has not changed is a good sign. However, to be safe, you should be carefully evaluated by a
board certified dermatologist. Please use the ABCDE's as your guide for
when to be concerned about moles: A for Asymmetry; B for
Irregular Borders; C for
Black or uneven Colors; D for diameter greater than a pencil eraser; and E for
a growth that is Evolving or changing. If necessary, it is an
extremely simple five minute procedure for a dermatologist to remove your mole
using a "shave" biopsy which typically
leaves a minimal scar. The mole can then be checked microscopically for
reassurance.You can follow the video link to learn more about identifying the
changes in moles that are suspicious for skin cancer.