Should I worry about dark spots on my dog's belly near his penis?

If the skin is not irritated/itchy and feels the same as the skin surrounding that is not dark in color, I would just monitor the area. We can have changes in pigmentation of the skin in dogs as they age, as well as changes as the result of inflammation. If the area begins to spread, or the skin feels dry/rough in this area, or she is licking, a visit to the vet is recommended.

Those black spots look like comedones (aka: blackheads). I suggest you use this shampoo that contains benzoyl peroxide. Lather the affected areas (no need to do the whole dog), let it sit for 10-15 minutes and then rinse throughly. Do this every 2-3 days until they are gone. Then use this shampoo 1-2 times every month for maintenance. ( allivet.com/p-7130-oxiderm-shampoo-ps.aspx )

That may be just normal pigmentation changes. If the skin feels, or looks rougher than normal, if there are scabs or if she itches those areas then I would be more concerned. I hope this was helpful!

This looks like normal pigmentation to me however it could also be post inflammatory hyperpigmentation from an infection. I wouldn’t be overly worried but I suggest you use these medicated wipes on the pigmented areas daily for 7-10 days. If no change then it’s either normal or you should have your vet check it. ( https://www.allivet.com/p-5393-douxo-chlorhexidine-3-ps-pads-for-dogs-and-cats-30.aspx )

Blue and purple spots could be bruises and if they are spreading I would be worried that your dog may have a coagulation problem. This would be very serious and would need to be investigated. A small blood test will find out whether your dog is either suffering from a congenital blood coagulation disorder or possibly ate some rodenticide which can cause the blood not to coagulate. Of course it could also just be hyperpigmentation. There are various causes for that, one being an endocrine disorder where the adrenal glands produce too much cortisone (cushings disease). I would recommend you make an appointment for an exam with your veterinarian.

This looks like a bruise, or possible bug bite. I recommend you photograph it every 3-4days for 7-10days to monitor if it is reducing or not. A full check up would be required if it is extending, changing colour or becomes painful or if it does not resolve within 3-4weeks.

This looks like hyperpigmentation. It typically occurs in lightly pigmented, sparsely haired regions as dogs get older. It can also be caused my inflammation so just make sure she is not spending too much time sunbathing and that there are no other signs like licking/scratching/lesions. I hope this helps and thank you for using Petco Pet Education Center, formerly Petcoach!