I have a Silkie hen who always gets black, stinky poop when she has the medicated Purina starter crumbles. We switched to Purina flock raiser, and she is once again gorgeous and has good poops.
Honestly I never found out what the "medicated" part is. Do they put antibiotics in it? So, since ours freerange out in the sun all day, they are encouraged to encounter coccidia(s) and develop immunity the average, live chicken normal way. So far, they're better and gorgeous and happy on the Purina flockraiser crumbles.
We only buy the tiny ziploc baggie of Purina starter, just for the first week or so of hatching. But that poor Silkie hen feeds it to her chicks and gets sick, then gets better a day after switching off the "medicated" starter crumbles. It's happened every time.