My dog has black skin spots, skin tags, and discharge. Should I worry?

Hello, & thank you for using Petco Pet Education Center, formerly Petcoach. It is always a good idea to seek a hands on veterinary visit, especially as a dog ages & new questions about health arise. Based on your pictures, it is possible that the discolored areas are indeed a reaction to the heat rash. Dog skin will often respond with the production of dark pigments ( melanin ) especially when a condition becomes chronic. As long as the area is not showing signs of a moist dermatitis & there is no significant redness or inflammation, then you can continue to treat the area topically. Skin tags are a common benign polyp, again fairly common as dogs age. The discharge also is probably a benign & normal accumulation of secretions. If each or any of the above become more concerning, then it really is in Nemo's best interest to have things thoroughly looked at & dealt with early on. Take care.