Black spot on dogs prepuce

Noticed a black growth on tip of dogs prepuce (I think is the word) He can still urinate fine. He is not neutered

Hello   Skin does not usually change its basal colour in dogs and when it does, such as in this case, we strongly reccommend a veterinary appointment.  Causes of colour change in skin can include infection (e.g. by yeasts), although this example looks surprisingly clearly demarcated for that.   The vet might therefore decide to initiate tests, for example a biopsy, in order to better pinpoint the cause.  Cancer ( for example SCC or mast cell tumours) is something that your vets may consider.