Critter crazy hit it spot on & read the link too, to understand exactly what this disease is and how it gets transmitted.
This disease goes through a host cycle, which includes earthworms and cecal worms. Chickens can get infected but the average chicken does not get ill from it.. instead they live on and so they shed the eggs and parasite into the soil, to be picked up by earthworms and so on to new birds. Turkeys and peafowl have very low resistance to this disease, which means it is fatal for almost all that get infected, if not treated.
Worming works because it breaks the host cycle by killing the cecal worms.. a creature the disease requires to complete the cycle. No cecals, blackhead can;t get in there and infect the bird.. So if you keep the turkeys(and peafowl) wormed regularly, it's a pretty good preventative. Not 100% fool proof but heck a whole better than never worming at all.
We're lucky to live in an area that is mostly dry so the chances is already low but still, be a good idea to worm anyways, just to be sure. If you have bought birds from auctions or had any adult or started birds shipped in from other states(page says blackhead is common in mideast and some east areas), the chances of having chicken infected with Blackhead can be pretty high.. so if you have such chickens, best worm everything.
If you are really concerned, it is best to have them far from each other- remember earthworms can get infected, they could then move to the turkey area and get eaten..
I've had mine for a couple years and didn't have problems, just tried to keep their pens as dry as possible plus regular worming. I do try to keep them in separate pens if possible, but have kept both together in same pen before (just not turkeys and peacocks together- they don't get along too well)
p.s. the reason why some people mix them together and "never had any problems" usually are because the black head disease was not on their premises in the first place.. just sheer luck or due to conditions that aren't favorable to earth worms such as the desert areas or regular liming/salting of the pens etc.. So CC's advise to call the Ag office would tell if it;s present in the general area.. but then if they say it;s not present in the general area and you bring in chickens from areas that DO have it.. well, you get the idea.. be careful of false comfort.