Posting to add to the collective group here that our son is 26 mo, two wks into potty training and this started occurring. Lots of logic on here in regards to new patterns / long holding / possible dehydration / peeing into a waterless mini potty/ etc. Perhaps just another odd toddler thing! (No other symptoms). Weird doctors haven’t picked up on this trend. Found a random online link here if helpful,
https://www.healthcheckup.com/general/what-causes-black-specks-in-urine/amp/. It notes that it could be caused by a range of things (uti, etc) but could just be “ In most of the cases, these are remains of cells in the urinary tract that fall off and get carried along with the urine.” not going to stress unless he had more symptoms.