Is black spot on penis dangerous?

"Black spot" isn't really very helpful. It could refer to a wide number of things.

A tiny black spot half the size of a grain of rice is probably a blackhead, caused by pores being blocked by dirt, it can be washed out and shows that you probably have poor hygiene.

If it's an ulcer spot (black crater in the skin with either pus, crust or liquid inside it) then it could be syphilis (an STD), herpes (also an STD) or penile cancer.

If it is a plaque (raised, flat-top lesions) it could be eczema or Balanitis (an inflammation condition).

Lastly if it is a papsule (small bump raised above the skin surface) then it can be a hair follicle, a wart or Psoriasis.

Ideally you should visit a doctor to get a professional diagnosis of it. No need to be embarrassed, they deal with penises every single day and have seen them all.