I think that the ink is pooling on the page, it is not being absorbed quickly and as its level build up the print head is spreading the pooled ink. I have had this problem with package of HP glossy in the past--not so much the spreading but the pooling. You indicate that this effect is occurring where you have a solid black bkgrnd--make sense--lots of ink in a given space.
You might be able to prevent this if you can choose a reduced black ink 'laydown' level in the print dialogue. I doubt that this will do--use the proper paper and move on.
god luck irv weiner