Its stains the roots man , B52 is super dark brown and odds are thats why its probly not black your seeing but a really really dark brown ! If it just rinses away i wouldnt be to scared man if when you do it your plants roots feel mushy you might have a problem but all in all i dont think that you do , I think that its just the "dye" from the different nutrient solutions mixed into one and its just staining your plants roots . because looking at your grow i dont see any problems which leads me to believe your root mass is fine and just stained . I hope this information helps you in understand whats happening :) -Happy Growing!