I have a sort of a small growth on my back that is blackish in colour. not painful.

Needs assessment: Best would be by your doc. While majority skin lesions are benign, there are instances where they pose a significant threat to individual's health. An assessment is recommended in person.

Need to examine: It is not feasible to provide a meaningful opinion without additional history, physical examination and may be some tests. You may watch and wait. If the mole/lesion doubles in size, becomes irregular in size, shape or color, bleeds, ulcerates or develops a nodule, see a doctor. Wish you good health!