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!

The only thing that comes to mind is that your new (tap) water if full of chlorine, try using distilled water or RO water, at the very least let the water that will go into the res sit 48 hours to evaporate most of the chlorine