black spots

Black spots, unless linked to something specifically that you might have ingested, might be a sign of an upper GI bleed. Do you have stomach pain/cramps or known history of stomach ulcers/acid reflux in addition to your UP? Blood, as it travels through the GI tract and after the passage of time, will darken in color and show up as such in the stool. Typically with active UC with active bleeding, the blood in our stool will be the more typical red color but it all depends upon the transit time moving through the GI tract. If there has been a break in time since an episode of diarrhea/blood/mucus than the blood will have time to darken in color and can show up as the black spots you describe.

34 yo male, physician
In remission from UC (pancolitis)
Currently following SCD, VSL#3 DS

The sharp pain in the lower left abdomen is indicative of distal disease (UP). The black spots could be blood. Your tag shows that you've flared a lot in the last year. The pentasa is probably not reaching the distal aspect of the large intestine well enough. You probably should try a rectal foam/enema to help control the inflammation.