black tips on roots is from oxygen exposure and not something to worry about too much, right now the babies in my grow have been in rockwool for prob about 3 weeks maybe 4 and they are getting blackened tips, the blackened tips are only on the root stock jutting out from the bottom of the cubes out the sides being exposed to oxygen and drying out, this happened in my first grow and i was freaking out, i xfered them to the clay pellets and then they were fine and i got killer results, as long as your ladies are looking okay as far as foliage goes they should be fine, probably just need transfered to your claypellets and buried.