Without a direct examination, it's difficult to say, but it doesn't sound like any typical symptom of an STD. Genital warts can be dark in color, but usually are either pink or brownish (not black or blood-colored). This area of the penis is prone to small cysts, which is what you're describing might be. Also, a certain type of papule called fordyce spots appear on the penis as well, and appear as small bright red or purple spots that are not painful or itchy. These are just dilated blood vessels covered by thickened skin and can bleed if injured (such as during intercourse or by squeezing). They are not infectious and not caused by any STD. The cause is unknown and they're considered a normal variant, but can sometimes be removed by electrocautery if the apperance is bothersome.
There's also a small chance that it could be a mole or a type of plaque or lesion that needs to be evaluated for malignancy. A visit to a dermatologist would be recommended to rule out anything serious or infectious.
I hope this helps!